Darwinism (Wallace)/Chapter XIV
|←Chapter XIII||Darwinism (Wallace) by
Chapter XIV - FUNDAMENTAL PROBLEMS IN RELATION TO VARIATION AND HEREDITY
FUNDAMENTAL PROBLEMS IN RELATION TO VARIATION AND HEREDITY
Fundamental difficulties and objections—Mr. Herbert Spencer's factors of organic evolution—Disuse and effects of withdrawal of natural selection—Supposed effects of disuse among wild animals—Difficulty as to co-adaptation of parts by variation and selection—Direct action of the environment—The American school of evolutionists—Origin of the feet of the ungulates—Supposed action of animal intelligence—Semper on the direct influence of the environment—Professor Geddes's theory of variation in plants—Objections to the theory—On the origin of spines—Variation and selection overpower the effects of use and disuse—Supposed action of the environment in imitating variations—Weismann's theory of heredity—The cause of variation—The non-heredity of acquired characters—The theory of instinct—Concluding remarks.
Having now set forth and illustrated at some length the most important of the applications of the development hypothesis in the explanation of the broader and more generally interesting phenomena presented by the organic world, we propose to discuss some of the more fundamental problems and difficulties which have recently been adduced by eminent naturalists. It is the more necessary to do this, because there is now a tendency to minimise the action of natural selection in the production of organic forms, and to set up in its place certain fundamental principles of variation or laws of growth, which it is urged are the real originators of the several lines of development, and of most of the variety of form and structure in the vegetable and animal kingdoms. These views have, moreover, been seized upon by popular writers to throw doubt and discredit on the whole theory of evolution, and especially on Darwin's presentation of that theory, to the bewilderment of the general public, who are quite unable to decide how far the new views, even if well established, tend to subvert the Darwinian theory, or whether they are really more than subsidiary parts of it, and quite powerless without it to produce any effect whatever.
The writers whose special views we now propose to consider are: (1) Mr. Herbert Spencer, on modification of structures arising from modification of functions, as set forth in his Factors of Organic Evolution. (2) Dr. E.D. Cope, who advocates similar views in detail, in his work entitled The Origin of the Fittest, and may be considered the head of a school of American naturalists who minimise the agency of natural selection. (3) Dr. Karl Semper, who has especially studied the direct influence of the environment in the whole animal kingdom, and has set forth his views in a volume on The Natural Conditions of Existence as they Affect Animal Life. (4) Mr. Patrick Geddes, who urges that fundamental laws of growth, and the antagonism of vegetative and reproductive forces, account for much that has been imputed to natural selection.
We will now endeavour to ascertain what are the more important facts and arguments adduced by each of the above writers, and how far they offer a substitute for the action of natural selection; having done which, a brief account will be given of the views of Dr. Aug. Weismann, whose theory of heredity will, if established, strike at the very root of the arguments of the first three of the writers above referred to.
Mr. Herbert Spencer's Factors of Organic Evolution.
Mr. Spencer, while fully recognising the importance and wide range of the principle of natural selection, thinks that sufficient weight has not been given to the effects of use and disuse as a factor in evolution, or to the direct action of the environment in determining or modifying organic structures. As examples of the former class of actions, he adduces the decreased size of the jaws in the civilised races of mankind, the inheritance of nervous disease produced by overwork, the great and inherited development of the udders in cows and goats, and the shortened legs, jaws, and snout in improved races of pigs—the two latter examples being quoted from Mr. Darwin,—and other cases of like nature. As examples of the latter, Mr. Darwin is again quoted as admitting that there are many cases in which the action of similar conditions appears to have produced corresponding changes in different species; and we have a very elaborate discussion of the direct action of the medium in modifying the protoplasm of simple organisms, so as to bring about the difference between the outer surface and the inner part that characterises the cells or other units of which they are formed.
Now, although this essay did little more than bring together facts which had been already adduced by Mr. Darwin or by Mr. Spencer himself, and lay stress upon their importance, its publication in a popular review was immediately seized upon as "an avowed and definite declaration against some of the leading ideas on which the Mechanical Philosophy depends," and as being "fatal to the adequacy of the Mechanical Philosophy as any explanation of organic evolution,"—an expression of opinion which would be repudiated by every Darwinian. For, even admitting the interpretation which Mr. Spencer puts on the facts he adduces, they are all included in the causes which Darwin himself recognised as having acted in bringing about the infinitude of forms in the organic world. In the concluding chapter of the Origin of Species he says: "I have now recapitulated the facts and considerations which have thoroughly convinced me that species have been modified during a long course of descent. This has been effected chiefly through the natural selection of numerous successive, slight, favourable variations; aided in an important manner by the inherited effects of the use and disuse of parts; and in an unimportant manner—that is, in relation to adaptive structures whether past or present, by the direct action of external conditions, and by variations which seem to us, in our ignorance, to arise spontaneously." This passage, summarising Darwin's whole inquiry, and explaining his final point of view, shows how very inaccurate may be the popular notion, as expressed by the Duke of Argyll, of any supposed additions to the causes of change of species as recognised by Darwin.
But, as we shall see presently, there is now much reason to believe that the supposed inheritance of acquired modifications—that is, of the effects of use and disuse, or of the direct influence of the environment—is not a fact; and if so, the very foundation is taken away from the whole class of objections on which so much stress is now laid. It therefore becomes important to inquire whether the facts adduced by Darwin, Spencer, and others, do really necessitate such inheritance, or whether any other interpretation of them is possible. I believe there is such an interpretation; and we will first consider the cases of disuse on which Mr. Spencer lays most stress.
The cases Mr. Spencer adduces as demonstrating the effects of disuse in diminishing the size and strength of organs are, the diminished size of the jaws in the races of civilised men, and the diminution of the muscles used in closing the jaws in the case of pet-dogs fed for generations on soft food. He argues that the minute reduction in any one generation could not possibly have been useful, and, therefore, not the subject of natural selection; and against the theory of correlation of the diminished jaw with increased brain in man, he urges that there are cases of large brain development, accompanied by jaws above the average size. Against the theory of economy of nutrition in the case of the pet-dogs, he places the abundant food of these animals which would render such economy needless.
But neither he nor Mr. Darwin has considered the effects of the withdrawal of the action of natural selection in keeping up the parts in question to their full dimensions, which, of itself, seems to me quite adequate to produce the results observed. Recurring to the evidence, adduced in Chapter III, of the constant variation occurring in all parts of the organism, while selection is constantly acting on these variations in eliminating all that fall below the best working standard, and preserving only those that are fully up to it; and, remembering further, that, of the whole number of the increase produced annually, only a small percentage of the best adapted can be preserved, we shall see that every useful organ will be kept up nearly to its higher limit of size and efficiency. Now Mr. Galton has proved experimentally that, when any part has thus been increased (or diminished) by selection, there is in the offspring a strong tendency to revert to a mean or average size, which tends to check further increase. And this mean appears to be, not the mean of the actual existing individuals but a lower mean, or that from which they had been recently raised by selection. He calls this the law of "Regression towards Mediocrity," and it has been proved by experiments with vegetables and by observations on mankind. This regression, in every generation, takes place even when both parents have been selected for their high development of the organ in question; but when there is no such selection, and crosses are allowed among individuals of every grade of development, the deterioration will be very rapid; and after a time not only will the average size of the part be greatly reduced, but the instances of full development will become very rare. Thus what Weismann terms "panmixia," or free intercrossing, will co-operate with Galton's law of "regression towards mediocrity," and the result will be that, whenever selection ceases to act on any part or organ which has heretofore been kept up to a maximum of size and efficiency, the organ in question will rapidly decrease till it reaches a mean value considerably below the mean of the progeny that has usually been produced each year, and very greatly below the mean of that portion which has survived annually; and this will take place by the general law of heredity, and quite irrespective of any use or disuse of the part in question. Now, no observations have been adduced by Mr. Spencer or others, showing that the average amount of change supposed to be due to disuse is greater than that due to the law of regression towards mediocrity; while even if it were somewhat greater, we can see many possible contributory causes to its production. In the case of civilised man's diminished jaw, there may well be some correlation between the jaw and the brain, seeing that increased mental activity would lead to the withdrawal of blood and of nervous energy from adjacent parts, and might thus lead to diminished growth of those parts in the individual. And in the case of pet-dogs, the selection of small or short-headed individuals would imply the unconscious selection of those with less massive temporal muscles, and thus lead to the concomitant reduction of those muscles. The amount of reduction observed by Darwin in the wing-bones of domestic ducks and poultry, and in the hind legs of tame rabbits, is very small, and is certainly no greater than the above causes will well account for; while so many of the external characters of all our domestic animals have been subject to long-continued artificial selection, and we are so ignorant of the possible correlations of different parts, that the phenomena presented by them seem sufficiently explained without recurrence to the assumption that any changes in the individual, due to disuse, are inherited by the offspring.
Supposed Effects of Disuse among Wild Animals.
It may be urged, however, that among wild animals we have many undoubted results of disuse much more pronounced than those among domestic kinds, results which cannot be explained by the causes already adduced. Such are the reduced size of the wings of many birds on oceanic islands; the abortion of the eyes in many cave animals, and in some which live underground; and the loss of the hind limbs in whales and in some lizards. These cases differ greatly in the amount of the reduction of parts which has taken place, and may be due to different causes. It is remarkable that in some of the birds of oceanic islands the reduction is little if at all greater than in domestic birds, as in the water-hen of Tristan d'Acunha. Now if the reduction of wing were due to the hereditary effects of disuse, we should expect a very much greater effect in a bird inhabiting an oceanic island than in a domestic bird, where the disuse has been in action for an indefinitely shorter period. In the case of many other birds, however—as some of the New Zealand rails and the extinct dodo of Mauritius—the wings have been reduced to a much more rudimentary condition, though it is still obvious that they were once organs of flight; and in these cases we certainly require some other causes than those which have reduced the wings of our domestic fowls. One such cause may have been of the same nature as that which has been so efficient in reducing the wings of the insects of oceanic islands—the destruction of those which, during the occasional use of their wings, were carried out to sea. This form of natural selection may well have acted in the case of birds whose powers of flight were already somewhat reduced, and to whom, there being no enemies to escape from, their use was only a source of danger. We may thus, perhaps, account for the fact that many of these birds retain small but useless wings with which they never fly; for, the wings having been reduced to this functionless condition, no power could reduce them further except correlation of growth or economy of nutrition, causes which only rarely come into play.
The complete loss of eyes in some cave animals may, perhaps, be explained in a somewhat similar way. Whenever, owing to the total darkness, they became useless, they might also become injurious, on account of their delicacy of organisation and liability to accidents and disease; in which case natural selection would begin to act to reduce, and finally abort them; and this explains why, in some cases, the rudimentary eye remains, although completely covered by a protective outer skin. Whales, like moas and cassowaries, carry us back to a remote past, of whose conditions we know too little for safe speculation. We are quite ignorant of the ancestral forms of either of these groups, and are therefore without the materials needful for determining the steps by which the change took place, or the causes which brought it about.
On a review of the various examples that have been given by Mr. Darwin and others of organs that have been reduced or aborted, there seems too much diversity in the results for all to be due to so direct and uniform a cause as the individual effects of disuse accumulated by heredity. For if that were the only or chief efficient cause, and a cause capable of producing a decided effect during the comparatively short period of the existence of animals in a state of domestication, we should expect to find that, in wild species, all unused parts or organs had been reduced to the smallest rudiments, or had wholly disappeared. Instead of this we find various grades of reduction, indicating the probable result of several distinct causes, sometimes acting separately, sometimes in combination, such as those we have already pointed out.
And if we find no positive evidence of disuse, acting by its direct effect on the individual, being transmitted to the offspring, still less can we find such evidence in the case of the use of organs. For here the very fact of use, in a wild state, implies utility, and utility is the constant subject for the action of natural selection; while among domestic animals those parts which are exceptionally used are so used in the service of man, and have thus become the subjects of artificial selection. Thus "the great and inherited development of the udders in cows and goats," quoted by Spencer from Darwin, really affords no proof of inheritance of the increase due to use, because, from the earliest period of the domestication of these animals, abundant milk-production has been highly esteemed, and has thus been the subject of selection; while there are no cases among wild animals that may not be better explained by variation and natural selection.
Difficulty as to Co-adaptation of Parts by Variation and Selection.
Mr. Spencer again brings forward this difficulty, as he did in his Principles of Biology twenty-five years ago, and urges that all the adjustments of bones, muscles, blood-vessels, and nerves which would be required during, for example, the development of the neck and fore-limbs of the giraffe, could not have been effected by "simultaneous fortunate spontaneous variations." But this difficulty is fully disposed of by the facts of simultaneous variation adduced in our third chapter, and has also been specially considered in Chapter VI, p. 127. The best answer to this objection may, perhaps, be found in the fact that the very thing said to be impossible by variation and natural selection has been again and again effected by variation and artificial selection. During the process of formation of such breeds as the greyhound or the bulldog, of the race-horse and carthorse, of the fantail pigeon or the otter-sheep, many co-ordinate adjustments have been produced; and no difficulty has occurred, whether the change has been effected by a single variation—as in the last case named—or by slow steps, as in all the others. It seems to be forgotten that most animals have such a surplus of vitality and strength for all the ordinary occasions of life that any slight superiority in one part can be at once utilised; while the moment any want of balance occurs, variations in the insufficiently developed parts will be selected to bring back the harmony of the whole organisation. The fact that, in all domestic animals, variations do occur, rendering them swifter or stronger, larger or smaller, stouter or slenderer, and that such variations can be separately selected and accumulated for man's purposes, is sufficient to render it certain that similar or even greater changes may be effected by natural selection, which, as Darwin well remarks, "acts on every internal organ, on every shade of constitututional difference, on the whole machinery of life." The difficulty as to co-adaptation of parts by variation and natural selection appears to me, therefore, to be a wholly imaginary difficulty which has no place whatever in the operations of nature.
Direct Action of the Environment.
Mr. Spencer's last objection to the wide scope given by Darwinians to the agency of natural selection is, that organisms are acted upon by the environment, which produces in them definite changes, and that these changes in the individual are transmitted by inheritance, and thus become increased in successive generations. That such changes are produced in the individual there is ample evidence, but that they are inherited independently of any form of selection or of reversion is exceedingly doubtful, and Darwin nowhere expresses himself as satisfied with the evidence. The two very strongest cases he mentions are the twenty-nine species of American trees which all differed in a corresponding way from their nearest European allies; and the American maize which became changed after three generations in Europe. But in the case of the trees the differences alleged may be partly due to correlation with constitutional peculiarities dependent on climate, especially as regards the deeper tint of the fading leaves and the smaller size of the buds and seeds in America than in Europe; while the less deeply toothed or serrated leaves in the American species are, in our present complete ignorance of the causes and uses of serration, quite as likely to be due to some form of adaptation as to any direct action of the climate. Again, we are not told how many of the allied species do not vary in this particular manner, and this is certainly an important factor in any conclusion we may form on the question.
In the case of the maize it appears that one of the more remarkable and highly selected American varieties was cultivated in Germany, and in three years nearly all resemblance to the original parent was lost; and in the sixth year it closely resembled a common European variety, but was of somewhat more vigorous growth. In this case no selection appears to have been practised, and the effects may have been due to that "reversion to mediocrity" which invariably occurs, and is more especially marked in the case of varieties which have been rapidly produced by artificial selection. It may be considered as a partial reversion to the wild or unimproved stock; and the same thing would probably have occurred, though perhaps less rapidly, in America itself. As this is stated by Darwin to be the most remarkable case known to him "of the direct and prompt action of climate on a plant," we must conclude that such direct effects have not been proved to be accumulated by inheritance, independently of reversion or selection.
The remaining part of Mr. Spencer's essay is devoted to a consideration of the hypothetical action of the environment on the lower organisms which consist of simple cells or formless masses of protoplasm; and he shows with great elaboration that the outer and inner parts of these are necessarily subject to different conditions; and that the outer actions of air or water lead to the formation of integuments, and sometimes to other definite modifications of the surface, whence arise permanent differences of structure. Although in these cases also it is very difficult to determine how much is due to direct modification by external agencies transmitted and accumulated by inheritance, and how much to spontaneous variations accumulated by natural selection, the probabilities in favour of the former mode of action are here greater, because there is no differentiation of nutritive and reproductive cells in these simple organisms; and it can be readily seen that any change produced in the latter will almost certainly affect the next generation. We are thus carried back almost to the origin of life, and can only vaguely speculate on what took place under conditions of which we know so little.
The American School of Evolutionists.
The tentative views of Mr. Spencer which we have just discussed, are carried much further, and attempts have been made to work them out in great detail, by many American naturalists, whose best representative is Dr. E.D. Cope of Philadelphia. This school endeavours to explain all the chief modifications of form in the animal kingdom by fundamental laws of growth and the inherited effects of use and effort, returning, in fact, to the teachings of Lamarck as being at least equally important with those of Darwin.
The following extract will serve to show the high position claimed by this school as original discoverers, and as having made important additions to the theory of evolution:
"Wallace and Darwin have propounded as the cause of modification in descent their law of natural selection. This law has been epitomised by Spencer as the 'survival of the fittest.' This neat expression no doubt covers the case, but it leaves the origin of the fittest entirely untouched. Darwin assumes a 'tendency to variation' in nature, and it is plainly necessary to do this, in order that materials for the exercise of a selection should exist. Darwin and Wallace's law is then only restrictive, directive, conservative, or destructive of something already created. I propose, then, to seek for the originative laws by which these subjects are furnished; in other words, for the causes of the origin of the fittest."
Mr. Cope lays great stress on the existence of a special developmental force termed "bathmism" or growth-force, which acts by means of retardation and acceleration "without any reference to fitness at all;" that "instead of being controlled by fitness it is the controller of fitness." He argues that "all the characteristics of generalised groups from genera up (excepting, perhaps, families) have been evolved under the law of acceleration and retardation," combined with some intervention of natural selection; and that specific characters, or species, have been evolved by natural selection with some assistance from the higher law. He, therefore, makes species and genera two absolutely distinct things, the latter not developed out of the former; generic characters and specific characters are, in his opinion, fundamentally different, and have had different origins, and whole groups of species have been simultaneously modified, so as to belong to another genus; whence he thinks it "highly probable that the same specific form has existed through a succession of genera, and perhaps in different epochs of geologic time."
Useful characters, he concludes, have been produced by the special location of growth-force by use; useless ones have been produced by location of growth-force without the influence of use. Another element which determines the direction of growth-force, and which precedes use, is effort; and "it is thought that effort becomes incorporated into the metaphysical acquisitions of the parent, and is inherited with other metaphysical qualities by the young, which, during the period of growth, is much more susceptible to modifying influences, and is likely to exhibit structural change in consequence."
From these few examples of their teachings, it is clear that these American evolutionists have departed very widely from the views of Mr. Darwin, and in place of the well-established causes and admitted laws to which he appeals have introduced theoretical conceptions which have not yet been tested by experiments or facts, as well as metaphysical conceptions which are incapable of proof. And when they come to illustrate these views by an appeal to palaeontology or morphology, we find that a far simpler and more complete explanation of the facts is afforded by the established principles of variation and natural selection. The confidence with which these new ideas are enunciated, and the repeated assertion that without them Darwinism is powerless to explain the origin of organic forms, renders it necessary to bestow a little more time on the explanations they give us of well-known phenomena with which, they assert, other theories are incompetent to grapple.
As examples of use producing structural change, Mr. Cope adduces the hooked and toothed beaks of the falcons and the butcher-birds, and he argues that the fact of these birds belonging to widely different groups proves that similarity of use has produced a similar structural result. But no attempt is made to show any direct causal connection between the use of a bill to cut or tear flesh and the development of a tooth on the mandible. Such use might conceivably strengthen the bill or increase its size, but not cause a special tooth-like outgrowth which was not present in the ancestral thrush-like forms of the butcher-bird. On the other hand, it is clear that any variations of the bill tending towards a hook or tooth would give the possessor some advantage in seizing and tearing its prey, and would thus be preserved and increased by natural selection. Again, Mr. Cope urges the effects of a supposed "law of polar or centrifugal growth" to counteract a tendency to unsymmetrical growth, where one side of the body is used more than the other. But the undoubted hurtfulness of want of symmetry in many important actions or functions would rapidly eliminate any such tendency. When, however, it has become useful, as in the case of the single enlarged claw of many Crustacea, it has been preserved by natural selection.
Origin of the Feet of the Ungulates.
Perhaps the most original and suggestive of Mr. Cope's applications of the theory of use and effort in modifying structure are, his chapters "On the Origin of the Foot-Structure of the Ungulates;" and that "On the Effect of Impacts and Strains on the Feet of Mammalia;" and they will serve also to show the comparative merits of this theory and that of natural selection in explaining a difficult case of modification, especially as it is an explanation claimed as new and original when first enunciated in 1881. Let us, then, see how he deals with the problem.
The remarkable progressive change of a four or five-toed ancestor into the one-toed horse, and the equally remarkable division of the whole group of ungulate animals into the odd-toed and even-toed divisions, Mr. Cope attempts to explain by the effects of impact and use among animals which frequented hard or swampy ground respectively. On hard ground, it is urged, the long middle toe would be most used and subjected to the greatest strains, and would therefore acquire both strength and development. It would then be still more exclusively used, and the extra nourishment required by it would be drawn from the adjacent less-used toes, which would accordingly diminish in size, till, after a long series of changes, the records of which are so well preserved in the American tertiary rocks, the true one-toed horse was developed. In soft or swampy ground, on the other hand, the tendency would be to spread out the foot so that there were two toes on each side. The two middle toes would thus be most used and most subject to strains, and would, therefore, increase at the expense of the lateral toes. There would be, no doubt, an advantage in these two functional toes being of equal size, so as to prevent twisting of the foot while walking; and variations tending to bring this about would be advantageous, and would therefore be preserved. Thus, by a parallel series of changes in another direction, adapted to a distinct set of conditions, we should arrive at the symmetrical divided hoofs of our deer and cattle. The fact that sheep and goats are specially mountain and rock-loving animals may be explained by their being a later modification, since the divided hoof once formed is evidently well adapted to secure a firm footing on rugged and precipitous ground, although it could hardly have been first developed in such localities. Mr. Cope thus concludes: "Certain it is that the length of the bones in the feet of the ungulate orders has a direct relation to the dryness of the ground they inhabit, and the possibility of speed which their habit permits them or necessarily imposes on them."
If there is any truth in the explanation here briefly summarised, it must entirely depend on the fact of individual modifications thus produced being hereditary, and we yet await the proof of this. In the meantime it is clear that the very same results could have been brought about by variation and natural selection. For the toes, like all other organs, vary in size and proportions, and in their degree of union or separation; and if in one group of animals it was beneficial to have the middle toe larger and longer, and in another set to have the two middle toes of the same size, nothing can be more certain than that these particular modifications would be continuously preserved, and the very results we see ultimately produced.
The oft-repeated objections that the cause of variations is unknown, that there must be something to determine variations in the right direction; that "natural selection includes no actively progressive principle, but must wait for the development of variation, and then, after securing the survival of the best, wait again for the best to project its own variations for selection," we have already sufficiently answered by showing that variation—in abundant or typical species—is always present in ample amount; that it exists in all parts and organs; that these vary, for the most part, independently, so that any required combination of variations can be secured; and finally, that all variation is necessarily either in excess or defect of the mean condition, and that, consequently, the right or favourable variations are so frequently present that the unerring power of natural selection never wants materials to work upon.
Supposed Action of Animal Intelligence.
The following passage briefly summarises Mr. Cope's position: "Intelligence is a conservative principle, and will always direct effort and use into lines which will be beneficial to its possessor. Here we have the source of the fittest, i.e. addition of parts by increase and location of growth-force, directed by the influence of various kinds of compulsion in the lower, and intelligent option among higher animals. Thus intelligent choice, taking advantage of the successive evolution of physical conditions, may be regarded as the originator of the fittest, while natural selection is the tribunal to which all results of accelerated growth are submitted. This preserves or destroys them, and determines the new points of departure on which accelerated growth shall build."
This notion of "intelligence"—the intelligence of the animal itself—determining its own variation, is so evidently a very partial theory, inapplicable to the whole vegetable kingdom, and almost so to all the lower forms of animals, amongst which, nevertheless, there is the very same adaptation and co-ordination of parts and functions as among the highest, that it is strange to see it put forward with such confidence as necessary for the completion of Darwin's theory. If "the various kinds of compulsion"—by which are apparently meant the laws of variation, growth, and reproduction, the struggle for existence, and the actions necessary to preserve life under the conditions of the animal's environment—are sufficient to have developed the varied forms of the lower animals and of plants, we can see no reason why the same "compulsion" should not have carried on the development of the higher animals also. The action of this "intelligent option" is altogether unproved; while the acknowledgment that natural selection is the tribunal which either preserves or destroys the variations submitted to it, seems quite inconsistent with the statement that intelligent choice is the "produced by some other power, then natural selection is not wanted. If, on the other hand, both fit and unfit are produced, and natural selection decides between them, that is pure Darwinism, and Mr. Cope's theories have added nothing to it.of the fittest," since whatever is really "the fittest" can never be destroyed by natural selection, which is but another name for the survival of the fittest. If "the fittest" is always definitely
Semper on the Direct Influence of the Environment.
Professor Geddes's Theory of Variation in Plants.
In a paper read before the Edinburgh Botanical Society in 1886 Mr. Patrick Geddes laid down the outlines of a fundamental theory of plant variation, which he has further extended in the article "Variation and Selection" in the Encyclopaedia Britannica, and in a paper read before the Linnaean Society but not yet published.
A theory of variation should deal alike with the origin of specific distinctions and with those vaster differences which characterise the larger groups, and he thinks it should answer such questions as—How an axis comes to be arrested to form a flower? how the various forms of inflorescence were evolved? how did perigynous or epigynous flowers arise from hypogynous flowers? and many others equally fundamental. Natural selection acting upon numerous accidental variations will not, he urges, account for such general facts as these, which must depend on some constant law of variation. This law he believes to be the well-known antagonism of vegetative and reproductive growth acting throughout the whole course of plant development; and he uses it to explain many of the most characteristic features of the structure of flowers and fruits.
Commencing with the origin of the flower, which all botanists agree in regarding as a shortened branch, he explains this shortening as an inevitable physiological fact, since the cost of the development of the reproductive elements is so great as necessarily to check vegetative growth. In the same manner the shortening of the inflorescence from raceme to spike or umbel, and thence to the capitulum or dense flower-head of the composite plants is brought about. This shortening, carried still further, produces the flattened leaf-like receptacle of Dorstenia, and further still the deeply hollowed fruity receptacle of the fig.
The flower itself undergoes a parallel modification due to a similar cause. It is formed by a series of modified leaves arranged round a shortened axis. In its earlier stages the number of these modified leaves is indefinite, as in many Ranunculaceae; and the axis itself is not greatly shortened, as in Myosurus. The first advance is to a definite number of parts and a permanently shortened axis, in the arrangement termed hypogynous, in which all the whorls are quite distinct from each other. In the next stage there is a further shortening of the central axis, leaving the outer portion as a ring on which the petals are inserted, producing the arrangement termed perigynous. A still further advance is made by the contraction of the axis, so as to leave the central part forming the ovary quite below the flower, which is then termed epigynous.
These several modifications are said to be parallel and definite, and to be determined by the continuous checking of vegetation by reproduction along what is an absolute groove of progressive change. This being the case, the importance of natural selection is greatly diminished. Instead of selecting and accumulating spontaneous indefinite variations, its function is to retard them after the stage of maximum utility has been independently reached. The same simple conception is said to unlock innumerable problems of vegetable morphology, large and small alike. It explains the inevitable development of gymnosperm into angiosperm by the checked vegetative growth of the ovule-bearing leaf or carpel; while such minor adaptations as the splitting fruit of the geranium or the cupped stigma of the pansy, can be no longer looked upon as achievements of natural selection, but must be regarded as naturally traceable to the vegetative checking of their respective types of leaf organ. Again, a detailed examination of spiny plants practically excludes the hypothesis of mammalian selection altogether, and shows spines to arise as an expression of the diminishing vegetativeness—in fact, the ebbing vitality of a species.
Objections to the Theory.
The theory here sketched out is enticing, and at first sight seems calculated to throw much light on the history of plant development; but on further consideration, it seems wanting in definiteness, while it is beset with difficulties at every step. Take first the shortening of the raceme into the umbel and the capitulum, said to be caused by arrest of vegetative growth, due to the antagonism of reproduction. If this were the whole explanation of the phenomenon, we should expect the quantity of seed to increase as this vegetative growth diminished, since the seed is the product of the reproductive energy of the plant, and its quantity the best measure of that energy. But is this the case? The ranunculus has comparatively few seeds, and the flowers are not numerous; while in the same order the larkspur and the columbine have far more seeds as well as more flowers, but there is no shortening of the raceme or diminution of the foliage, although the flowers are large and complex. So, the extremely shortened and compressed flower-heads of the compositae produce comparatively few seeds—one only to each flower; while the foxglove, with its long spike of showy flowers, produces an enormous number.
Again, if the shortening of the central axis in the successive stages of hypogynous, perigynous, and epigynous flowers were an indication of preponderant reproduction and diminished vegetation, we should find everywhere some clear indications of this fact. The plants with hypogynous flowers should, as a rule, have less seed and more vigorous and abundant foliage than those at the other extreme with epigynous flowers. But the hypogynous poppies, pinks, and St. John's worts have abundance of seed and rather scanty foliage; while the epigynous dogwoods and honeysuckles have few seeds and abundant foliage. If, instead of the number of the seeds, we take the size of the fruit as an indication of reproductive energy, we find this at a maximum in the gourd family, yet their rapid and luxuriant growth shows no diminution of vegetative power. So that the statement that plant modifications proceed "along an absolute groove of progressive change" is contradicted by innumerable facts indicating advance and regression, improvement or degradation, according as the ever-changing environment renders one form more advantageous than the other. As one instance I may mention the Anonaceae or custard-apple tribe, which are certainly an advance from the Ranunculaceae; yet in the genus Polyalthea the fruit consists of a number of separate carpels, each borne on a long stalk, as if reverting to the primitive stalked carpellary leaves.
On the Origin of Spines.
But perhaps the most extraordinary application of the theory is that which considers spines to be an indication of the "ebbing vitality of a species," and which excludes "mammalian selection altogether." If this were true, spines should occur mainly in feeble, rare, and dying-out species, instead of which we have the hawthorn, one of our most vigorous shrubs or trees, with abundant vitality and an extensive range over the whole Palaearctic region, showing that it is really a dominant species. In North America the numerous thorny species of Crataegus are equally vigorous, as are the false acacia (Robinia) and the honey-locust (Gleditschia). Neither have the numerous species of very spiny Acacias been noticed to be rarer or less vigorous than the unarmed kinds.
On the other point—that spines are not due to mammalian selection—we are able to adduce what must be considered direct and conclusive evidence. For if spines, admittedly produced by aborted branches, petioles, or peduncles, are due solely or mainly to diminished vegetativeness or ebbing vitality, they ought to occur in all countries alike, or at all events in all whose similar conditions tend to check vegetation; whereas, if they are, solely or mainly, developed as a protection against the attacks of herbivorous mammals, they ought to be most abundant where these are plentiful, and rare or absent where indigenous mammalia are wanting. Oceanic islands, as compared with continents, would thus furnish a crucial test of the two theories; and Mr. Hemsley of Kew, who has specially studied insular floras, has given me some valuable information on this point. He says: "There are no spiny or prickly plants in the indigenous element of the St. Helena flora. The relatively rich flora of the Sandwich Isles is not absolutely without a prickly plant, but almost so. All the endemic genera are unarmed, and the endemic species of almost every other genus. Even such genera as Zanthoxylon, Acacia, Xylosoma, Lycium, and Solanum, of which there are many armed species in other countries, are only represented by unarmed species. The two endemic Rubi have the prickles reduced to the setaceous condition, and the two palms are unarmed.
"The flora of the Galapagos includes a number of prickly plants, among them several cacti (these have not been investigated and may be American species), but I do not think one of the known endemic species of any family is prickly or spiny.
"Spiny and prickly plants are also rare in New Zealand, but there are the formidably armed species of wild Spaniard (Aciphylla), one species of Rubus, the pungent-leaved Epacrideae and a few others."
Mr. J.G. Baker of Kew, who has specially studied the flora of Mauritius and the adjacent islands, also writes me on this point. He says: "Taking Mauritius alone, I do not call to mind a single species that is a spinose endemic tree or shrub. If you take the whole group of islands (Mauritius, Bourbon, Seychelles, and Rodriguez), there will be about a dozen species, but then nine of these are palms. Leaving out palms, the trees and shrubs of that part of the world are exceptionally non-spinose."
These are certainly remarkable facts, and quite inexplicable on the theory of spines being caused solely by checked vegetative growth, due to weakness of constitution or to an arid soil and climate. For the Galapagos and many parts of the Sandwich Islands are very arid, as is a considerable part of the North Island of New Zealand. Yet in our own moist climate and with our very limited number of trees and shrubs we have about eighteen spiny or prickly species, more, apparently, than in the whole endemic floras of the Mauritius, Sandwich Islands, and Galapagos, though these are all especially rich in shrubby and arboreal species. In New Zealand the prickly Rubus is a leafless trailing plant, and its prickles are probably a protection against the large snails of the country, several of which have shells from two to three and a half inches long. The "wild Spaniards" are very spiny herbaceous Umbelliferae, and may have gained their spines to preserve them from being trodden down or eaten by the Moas, which, for countless ages, took the place of mammals in New Zealand. The exact use or meaning of the spines in palms is more doubtful, though they are, no doubt, protective against some animals; but it is certainly an extraordinary fact that in the entire flora of the Mauritius, so largely consisting of trees and shrubs, not a single endemic species should be thorny or spiny.
If now we consider that every continental flora produces a considerable proportion of spiny and thorny species, and that these rise to a maximum in South Africa, where herbivorous mammalia were (before the settlement of the country), perhaps, more abundant and varied than in any other part of the world; while another district, remarkable for well-armed vegetation, is Chile, where the camel-like vicugnas, llamas, and alpacas, and an abundance of large rodents wage perpetual war against shrubby vegetation, we shall see the full significance of the almost total absence of thorny and spiny plants in the chief oceanic islands; and so far from "excluding the hypothesis of mammalian selection altogether," we shall find in this hypothesis the only satisfactory explanation of the facts.
From the brief consideration of Professor Geddes's theory now given, we conclude that, although the antagonism between vegetative and reproductive growth is a real agency, and must be taken account of in our endeavour to explain many of the fundamental facts in the structure and form of plants, yet it is so overpowered and directed at every step by the natural selection of favourable variations, that the results of its exclusive and unmodified action are nowhere to be found in nature. It may be allowed to rank as one of those "laws of growth," of which so many have now been indicated, and which were always recognised by Darwin as underlying all variation; but unless we bear in mind that its action must always be subordinated to natural selection, and that it is continually checked, or diverted, or even reversed by the necessity of adaptation to the environment, we shall be liable to fall into such glaring errors as the imputing to "ebbing vitality" alone such a widespread phenomenon as the occurrence of spines and thorns, while ignoring altogether the influence of the organic environment in their production.
The sketch now given of the chief attempts that have been made to prove that either the direct action of the environment or certain fundamental laws of variation are independent causes of modification of species, shows us that their authors have, in every case, failed to establish their contention. Any direct action of the environment, or any characters acquired by use or disuse, can have no effect whatever upon the race unless they are inherited; and that they are inherited in any case, except when they directly affect the reproductive cells, has not been proved. On the other hand, as we shall presently show, there is much reason for believing that such acquired characters are in their nature non-heritable.
Variation and Selection Overpower the Effects of Use and Disuse.
But there is another objection to this theory arising from the very nature of the effects produced. In each generation the effects of use or disuse, or of effort, will certainly be very small, while of this small effect it is not maintained that the whole will be always inherited by the next generation. How small the effect is we have no means of determining, except in the case of disuse, which Mr. Darwin investigated carefully. He found that in twelve fancy breeds of pigeons, which are often kept in aviaries, or if free fly but little, the sternum had been reduced by about one-seventh or one-eighth of its entire length, and that of the scapula about one-ninth. In domestic ducks the weight of the wing-bones in proportion to that of the whole skeleton had decreased about one-tenth. In domestic rabbits the bones of the legs were found to have increased in weight in due proportion to the increased weight of the body, but those of the hind legs were rather less in proportion to those of the fore legs than in the wild animal, a difference which may be imputed to their being less used in rapid motion. The pigeons, therefore, afford the greatest amount of reduction by disuse—one-seventh of the length of the sternum. But the pigeon has certainly been domesticated four or five thousand years; and if the reduction of the wings by disuse has only been going on for the last thousand years, the amount of reduction in each generation would be absolutely imperceptible, and quite within the limits of the reduction due to the absence of selection, as already explained. But, as we have seen in Chapter III, the fortuitous variation of every part or organ usually amounts to one-tenth, and often to one-sixth of the average dimensions—that is, the fortuitous variation in one generation among a limited number of the individuals of a species is as great as the cumulative effects of disuse in a thousand generations! If we assume that the effects of use or of effort in the individual are equal to the effects of disuse, or even ten or a hundred times greater, they will even then not will even then not equal, in each generation, the amount of the fortuitous variations of the same part. If it be urged that the effects of use would modify all the individuals of a species, while the fortuitous variations to the amount named only apply to a portion of them, it may be replied, that that portion is sufficiently large to afford ample materials for selection, since it often equals the numbers that can annually survive; while the recurrence in each successive generation of a like amount of variation would render possible such a rapid adjustment to new conditions that the effects of use or disuse would be as nothing in comparison. It follows, that even admitting the modifying effects of the environment, and that such modifications are inherited, they would yet be entirely swamped by the greater effects of fortuitous variation, and the far more rapid cumulative results of the selection of such variations.
Supposed Action of the Environment in Initiating Variations.
It is, however, urged that the reaction of the environment initiates variations, which without it would never arise; such, for instance, as the origin of horns through the pressures and irritations caused by butting, or otherwise using the head as a weapon or for defence. Admitting, for the sake of argument, that this is so, all the evidence we possess shows that, from the very first appearance of the rudiment of such an organ, it would vary to a greater extent than the amount of growth directly produced by use; and these variations would be subject to selection, and would thus modify the organ in ways which use alone would never bring about. We have seen that this has been the case with the branching antlers of the stag, which have been modified by selection, so as to become useful in other ways than as a mere weapon; and the same has almost certainly been the case with the variously curved and twisted horns of antelopes. In like manner, every conceivable rudiment would, from its first appearance, be subject to the law of variation and selection, to which, thenceforth, the direct effect of the environment would be altogether subordinate.
A very similar mode of reasoning will apply to the other branch of the subject—the initiation of structures and organs by the action of the fundamental laws of growth. Admitting that such laws have determined some of the main divisions of the animal and vegetable kingdom, have originated certain important organs, and have been the fundamental cause of certain lines of development, yet at every step of the process these laws must have acted in entire subordination to the law of natural selection. No modification thus initiated could have advanced a single step, unless it were, on the whole, a useful modification; while its entire future course would be necessarily subject to the laws of variation and selection, by which it would be sometimes checked, sometimes hastened on, sometimes diverted to one purpose, sometimes to another, according as the needs of the organism, under the special conditions of its existence, required such modification. We need not deny that such laws and influences may have acted in the manner suggested, but what we do deny is that they could possibly escape from the ever-present and all-powerful modifying effects of variation and natural selection.
Weismann's Theory of Heredity.
Professor August Weismann has put forth a new theory of heredity founded upon the "continuity of the germ-plasm," one of the logical consequences of which is, that acquired characters of whatever kind are not transmitted from parent to offspring. As this is a matter of vital importance to the theory of natural selection, and as, if well founded, it strikes away the foundations of most of the theories discussed in the present chapter, a brief outline of Weismann's views must be attempted, although it is very difficult to make them intelligible to persons unfamiliar with the main facts of modern embryology.
The problem is thus stated by Weismann: "How is it that in the case of all higher animals and plants a single cell is able to separate itself from amongst the millions of most various kinds of which an organism is composed, and by division and complicated differentiation to reconstruct a new individual with marvellous likeness, unchanged in many cases even throughout whole geological periods?" Darwin attempted to solve the problem by his theory of "Pangenesis," which supposed that every individual cell in the body gave off gemmules or germs capable of reproducing themselves, and that portions of these germs of each of the almost infinite number of cells permeate the whole body and become collected in the generative cells, and are thus able to reproduce the whole organism. This theory is felt to be so ponderously complex and difficult that it has met with no general acceptance among physiologists.
The fact that the germ-cells do reproduce with wonderful accuracy not only the general characters of the species, but many of the individual characteristics of the parents or more remote ancestors, and that this process is continued from generation to generation, can be accounted for, Weismann thinks, only on two suppositions which are physiologically possible. Either the substance of the parent germ-cell, after passing through a cycle of changes required for the construction of a new individual, possesses the capability of producing anew germ-cells identical with those from which that individual was developed, or the new germ-cells arise, as far as their essential and characteristic substance is concerned, not at all out of the body of the individual, but direct from the parent germ-cell. This latter view Weismann holds to be the correct one, and, on this theory, heredity depends on the fact that a substance of special molecular composition passes over from one generation to another. This is the "germ-plasm," the power of which to develop itself into a perfect organism depends on the extraordinary complication of its minutest structure. At every new birth a portion of the specific germ-plasm, which the parent egg-cell contains, is not used up in producing the offspring, but is reserved unchanged to produce the germ-cells of the following generation. Thus the germ-cells—so far as regards their essential part the germ-plasm—are not a product of the body itself, but are related to one another in the same way as are a series of generations of unicellular organisms derived from one another by a continuous course of simple division. Thus the question of heredity is reduced to one of growth. A minute portion of the very same germ-plasm from which, first the germ-cell, and then the whole organism of the parent, were developed, becomes the starting-point of the growth of the child.
The Cause of Variation.
But if this were all, the offspring would reproduce the parent exactly, in every detail of form and structure; and here we see the importance of sex, for each new germ grows out of the united germ-plasms of two parents, whence arises a mingling of their characters in the offspring. This occurs in each generation; hence every individual is a complex result reproducing in ever-varying degrees the diverse characteristics of his two parents, four grandparents, eight great-grandparents, and other more remote ancestors; and that ever-present individual variation arises which furnishes the material for natural selection to act upon. Diversity of sex becomes, therefore, of primary importance as the cause of variation. Where asexual generation prevails, the characteristics of the individual alone are reproduced, and there are thus no means of effecting the change of form or structure required by changed conditions of existence. Under such changed conditions a complex organism, if only asexually propagated, would become extinct. But when a complex organism is sexually propagated, there is an ever-present cause of change which, though slight in any one generation, is cumulative, and under the influence of selection is sufficient to keep up the harmony between the organism and its slowly changing environment.
The Non-Heredity of Acquired Characters.
Certain observations on the embryology of the lower animals are held to afford direct proof of this theory of heredity, but they are too technical to be made clear to ordinary readers. A logical result of the theory is the impossibility of the transmission of acquired characters, since the molecular structure of the germ-plasm is already determined within the embryo; and Weismann holds that there are no facts which really prove that acquired characters can be inherited, although their inheritance has, by most writers, been considered so probable as hardly to stand in need of direct proof.
We have already shown, in the earlier part of this chapter, that many instances of change, imputed to the inheritance of acquired variations, are really cases of selection; while the very fact that use implies usefulness renders it almost impossible to eliminate the action of selection in a state of nature. As regards mutilations, it is generally admitted that they are not hereditary, and there is ample evidence on this point. When it was the fashion to dock horses' tails, it was not found that horses were born with short tails; nor are Chinese women born with distorted feet; nor are any of the numerous forms of racial mutilation in man, which have in some cases been carried on for hundreds of generations, inherited. Nevertheless, a few cases of apparent inheritance of mutilations have been recorded, and these, if trustworthy, are difficulties in the way of the theory. The undoubted inheritance of disease is hardly a difficulty, because the predisposition to disease is a congenital, not an acquired character, and as such would be the subject of inheritance. The often-quoted case of a disease induced by mutilation being inherited (Brown-Sequard's epileptic guinea-pigs) has been discussed by Professor Weismann, and shown to be not conclusive. The mutilation itself—a section of certain nerves—was never inherited, but the resulting epilepsy, or a general state of weakness, deformity, or sores, was sometimes inherited. It is, however, possible that the mere injury introduced and encouraged the growth of certain microbes, which, spreading through the organism, sometimes reached the germ-cells, and thus transmitted a diseased condition to the offspring. Such a transference of microbes is believed to occur in syphilis and tuberculosis, and has been ascertained to occur in the case of the muscardine silkworm disease.
The Theory of Instinct.
The theory now briefly outlined cannot be said to be proved, but it commends itself to many physiologists as being inherently probable, and as furnishing a good working hypothesis till displaced by a better. We cannot, therefore, accept any arguments against the agency of natural selection which are based upon the opposite and equally unproved theory that acquired characters are inherited; and as this applies to the whole school of what may be termed Neo-Lamarckians, their speculations cease to have any weight.
The same remark applies to the popular theory of instincts as being inherited habits; though Darwin gave very little weight to this, but derived almost all instincts from spontaneous useful variations which, like other spontaneous variations, are of course inherited. At first sight it appears as if the acquired habits of our trained dogs—pointers, retrievers, etc.—are certainly inherited; but this need not be the case, because there must be some structural or psychical peculiarities, such as modifications in the attachments of muscles, increased delicacy of smell or sight, or peculiar likes and dislikes, which are inherited; and from these, peculiar habits follow as a natural consequence, or are easily acquired. Now, as selection has been constantly at work in improving all our domestic animals, we have unconsciously modified the structure, while preserving only those animals which best served our purpose in their peculiar faculties, instincts, or habits.
Much of the mystery of instinct arises from the persistent refusal to recognise the agency of imitation, memory, observation, and reason as often forming part of it. Yet there is ample evidence that such agency must be taken into account. Both Wilson and Leroy state that young birds build inferior nests to old ones, and the latter author observes that the best nests are made by birds whose young remain longest in the nest. So, migration is now well ascertained to be effected by means of vision, long flights being made on bright moonlight nights when the birds fly very high, while on cloudy nights they fly low, and then often lose their way. Thousands annually fly out to sea and perish, showing that the instinct to migrate is imperfect, and is not a good substitute for reason and observation.
Again, much of the perfection of instinct is due to the extreme severity of the selection during its development, any failure involving destruction. The chick which cannot break the eggshell, the caterpillar that fails to suspend itself properly or to spin a safe cocoon, the bees that lose their way or that fail to store honey, inevitably perish. So the birds that fail to feed and protect their young, or the butterflies that lay their eggs on the wrong food-plant, leave no offspring, and the race with imperfect instincts perishes. Now, during the long and very slow course of development of each organism, this rigid selection at every step of progress has led to the preservation of every detail of structure, faculty, or habit that has been necessary for the preservation of the race, and has thus gradually built up the various instincts which seem so marvellous to us, but which can yet be shown to be in many cases still imperfect. Here, as everywhere else in nature, we find comparative, not absolute perfection, with every gradation from what is clearly due to imitation or reason up to what seems to us perfect instinct—that in which a complex action is performed without any previous experience or instruction.
Having now passed in review the more important of the recent objections to, or criticisms of, the theory of natural selection, we have arrived at the conclusion that in no one case have the writers in question been able materially to diminish its importance, or to show that any of the laws or forces to which they appeal can act otherwise than in strict subordination to it. The direct action of the environment as set forth by Mr. Herbert Spencer, Dr. Cope, and Dr. Karl Semper, even if we admit that its effects on the individual are transmitted by inheritance, are so small in comparison with the amount of spontaneous variation of every part of the organism that they must be quite overshadowed by the latter. And if such direct action may, in some cases, have initiated certain organs or outgrowths, these must from their very first beginnings have been subject to variation and natural selection, and their further development have been almost wholly due to these ever-present and powerful causes. The same remark applies to the views of Professor Geddes on the laws of growth which have determined certain essential features in the morphology of plants and animals. The attempt to substitute these laws for those of variation and natural selection has failed in cases where we can apply a definite test, as in that of the origin of spines on trees and shrubs; while the extreme diversity of vegetable structure and form among the plants of the same country and of the same natural order, of itself affords a proof of the preponderating influence of variation and natural selection in keeping the many diverse forms in harmony with the highly complex and ever-changing environment.
Lastly, we have seen that Professor Weismann's theory of the continuity of the germ-plasm and the consequent non-heredity of acquired characters, while in perfect harmony with all the well-ascertained facts of heredity and development, adds greatly to the importance of natural selection as the one invariable and ever-present factor in all organic change, and that which can alone have produced the temporary fixity combined with the secular modification of species. While admitting, as Darwin always admitted, the co-operation of the fundamental laws of growth and variation, of correlation and heredity, in determining the direction of lines of variation or in the initiation of peculiar organs, we find that variation and natural selection are ever-present agencies, which take possession, as it were, of every minute change originated by these fundamental causes, check or favour their further development, or modify them in countless varied ways according to the varying needs of the organism. Whatever other causes have been at work, Natural Selection is supreme, to an extent which even Darwin himself hesitated to claim for it. The more we study it the more we are convinced of its overpowering importance, and the more confidently we claim, in Darwin's own words, that it "has been the most important, but not the exclusive, means of modification."
- See the Duke of Argyll's letter in Nature, vol. xxxiv. p. 336.
- Journal of the Anthropological Institute, vol. xv. pp. 246-260.
- The idea of the non-heredity of acquired variations was suggested by the summary of Professor Weismann's views, in Nature, referred to later on. But since this chapter was written I have, through the kindness of Mr. E.B. Poulton, seen some of the proofs of the forthcoming translation of Weismann's Essays on Heredity, in which he sets forth an explanation very similar to that here given. On the difficult question of the almost entire disappearance of organs, as in the limbs of snakes and of some lizards, he adduces "a certain form of correlation, which Roux calls 'the struggle of the parts in the organism,'" as playing an important part. Atrophy following disuse is nearly always attended by the corresponding increase of other organs: blind animals possess more developed organs of touch, hearing, and smell; the loss of power in the wings is accompanied by increased strength of the legs, etc. Now as these latter characters, being useful, will be selected, it is easy to understand that a congenital increase of these will be accompanied by a corresponding congenital diminution of the unused organ; and in cases where the means of nutrition are deficient, every diminution of these useless parts will be a gain to the whole organism, and thus their complete disappearance will, in some cases, be brought about directly by natural selection. This corresponds with what we know of these rudimentary organs.
It must, however, be pointed out that the non-heredity of acquired characters was maintained by Mr. Francis Galton more than twelve years ago, on theoretical considerations almost identical with those urged by Professor Weismann; while the insufficiency of the evidence for their hereditary transmission was shown, by similar arguments to those used above and in the work of Professor Weismann already referred to (see "A Theory of Heredity," in Journ. Anthrop. Instit., vol. v. pp. 343-345).
- This explanation is derived from Weismann's Theory of the Continuity of the Germ-Plasm as summarised in Nature.
- See a collection of his essays under the title, The Origin of the Fittest: Essays on Evolution, D. Appleton and Co. New York. 1887.
- Origin of the Fittest, p. 174.
- Ibid. p. 29. It may be here noted that Darwin found these theories unintelligible. In a letter to Professor E.T. Morse in 1877, he writes: "There is one point which I regret you did not make clear in your Address, namely, what is the meaning and importance of Professors Cope and Hyatt's views on acceleration and retardation? I have endeavoured, and given up in despair, the attempt to grasp their meaning" (Life and Letters, vol. iii. p. 233).
- Origin of the Fittest, p. 374.
- Origin of the Fittest, p. 40.
- The Natural Conditions of Existence as they Affect Animal Life. London, 1883.
- In Dr. Weismann's essay on "Heredity," already referred to, he considers it not improbable that changes in organisms produced by climatic influences may be inherited, because, as these changes do not affect the external parts of an organism only, but often, as in the case of warmth or moisture permeate the whole structure, they may possibly modify the germ-plasm itself, and thus induce variations in the next generation. In this way, he thinks, may possibly be explained the climatic varieties of certain butterflies, and some other changes which seem to be effected by change of climate in a few generations.
- This brief indication of Professor Geddes's views is taken from the article "Variation and Selection" in the Encyclopedia Britannica, and a paper "On the Nature and Causes of Variation in Plants" in Trans. and Proc. of the Edinburgh Botanical Society, 1886; and is, for the most part, expressed in his own words.
- Placostylis bovinus, 3½ inches long; Paryphanta Busbyi, 3 in. diam.; P. Hochstetteri, 2¾ in. diam.
- The general arguments and objections here set forth will apply with equal force to Professor G. Henslow's theory of the origin of the various forms and structures of flowers as due to "the responsive actions of the protoplasm in consequence of the irritations set up by the weights, pressures, thrusts, tensions, etc., of the insect visitors" (The Origin of Floral Structures through Insect and other Agencies, p. 340). On the assumption that acquired characters are inherited, such irritations may have had something to do with the initiation of variations and with the production of certain details of structure, but they are clearly incompetent to have brought about the more important structural and functional modifications of flowers. Such are, the various adjustments of length and position of the stamens to bring the pollen to the insect and from the insect to the stigma; the various motions of stamens and styles at the right time and the right direction; the physiological adjustments bringing about fertility or sterility in heterostyled plants; the traps, springs, and complex movements of various parts of orchids; and innumerable other remarkable phenomena.
For the explanation of these we have no resource but variation and selection, to the effects of which, acting alternately with regression or degradation as above explained (p. 328) must be imputed the development of the countless floral structures we now behold. Even the primitive flowers, whose initiation may, perhaps, have been caused, or rendered possible, by the irritation set up by insects' visits, must, from their very origin, have been modified, in accordance with the supreme law of utility, by means of variation and survival of the fittest.
- In an essay on "The Duration of Life," forming part of the translation of Dr. Weismann's papers already referred to, the author still further extends the sphere of natural selection by showing that the average duration of life in each species has been determined by it. A certain length of life is essential in order that the species may produce offspring sufficient to ensure its continuance under the most unfavourable conditions; and it is shown that the remarkable inequalities of longevity in different species and groups may be thus accounted for. Yet more, the occurrence of death in the higher organisms, in place of the continued survival of the unicellular organisms however much they may increase by subdivision, may be traced to the same great law of utility for the race and survival of the fittest. The whole essay is of exceeding interest, and will repay a careful perusal. A similar idea occurred to the present writer about twenty years back, and was briefly noted down at the time, but subsequently forgotten.
- The outline here given is derived from two articles in Nature, vol. xxxiii. p. 154, and vol. xxxiv. p. 629, in which Weismann's papers are summarised and partly translated.
- There are many indications that this explanation of the cause of variation is the true one. Mr. E.B. Poulton suggests one, in the fact that parthenogenetic reproduction only occurs in isolated species, not in groups of related species; as this shows that parthenogenesis cannot lead to the evolution of new forms. Again, in parthenogenetic females the complete apparatus for fertilisation remains unreduced; but if these varied as do sexually produced animals, the organs referred to, being unused, would become rudimentary.
Even more important is the significance of the "polar bodies," as explained by Weismann in one of his Essays; since, if his interpretation of them be correct, variability is a necessary consequence of sexual generation.
- Darwin's Animals and Plants, vol. ii. pp. 23, 24.
- In his essay on "Heredity," Dr. Weismann discusses many other cases of supposed inheritance of acquired characters, and shows that they can all be explained in other ways. Shortsightedness among civilised nations, for example, is due partly to the absence of selection and consequent regression towards a mean, and partly to its individual production by constant reading.
- Weismann explains instinct on similar lines, and gives many interesting illustrations (see Essays on Heredity). He holds "that all instinct is entirely due to the operation of natural selection, and has its foundation, not upon inherited experiences, but upon variations of the germ." Many interesting and difficult cases of instinct are discussed by Darwin in Chapter VIII of the Origin of Species, which should be read in connection with the above remarks.
Since this chapter was written my attention has been directed to Mr. Francis Galton's Theory of Heredity (already referred to at p. 417) which was published thirteen years ago as an alternative for Darwin's theory of pangenesis.
Mr. Galton's theory, although it attracted little attention, appears to me to be substantially the same as that of Professor Weismann. Galton's "stirp" is Weismann's "germ-plasm." Galton supposes the sexual elements in the offspring to be directly formed from the residue of the stirp not used up in the development of the body of the parent—Weismann's "continuity of the germ-plasm." Galton also draws many of the same conclusions from his theory. He maintains that characters acquired by the individual as the result of external influences cannot be inherited, unless such influences act directly on the reproductive elements—instancing the possible heredity of alcoholism, because the alcohol permeates the tissues and may reach the sexual elements. He discusses the supposed heredity of effects produced by use or disuse, and explains them much in the same manner as does Weismann. Galton is an anthropologist, and applies the theory, mainly, to explain the peculiarities of hereditary transmission in man, many of which peculiarities he discusses and elucidates. Weismann is a biologist, and is mostly concerned with the application of the theory to explain variation and instinct, and to the further development of the theory of evolution. He has worked it out more thoroughly, and has adduced embryological evidence in its support; but the views of both writers are substantially the same, and their theories were arrived at quite independently. The names of Galton and Weismann should therefore be associated as discoverers of what may be considered (if finally established) the most important contribution to the evolution theory since the appearance of the Origin of Species.