Popular Science Monthly/Volume 43/August 1893/Professor Weismann's Theories
|PROFESSOR WEISMANN'S THEORIES.|
APART from those more special theories of Prof. Weismann I lately dealt with, the wide acceptance of which by the biological world greatly surprises me, there are certain more general theories of his—fundamental theories—the acceptance of which surprises me still more. Of the two on which rests the vast superstructure of his speculation, the first concerns the distinction between the reproductive elements of each organism and the nonreproductive elements. He says:
"The answer to this question is closely bound up with the principle of division of labor which appeared among multicellular organisms at a very early stage. . . .
"The first multicellular organism was probably a cluster of similar cell?, but these units soon lost their original homogeneity. As the result of mere relative position, some of the cells were especially fitted to provide for the nutrition of the colony, while others undertook the work of reproduction" (Essays upon Heredity, p. 27).
Here, then, we have the great principle of the division of labor, which is the principle of all organization, taken as primarily illustrated in the division between the reproductive cells and the nonreproductive or somatic cells—the cells devoted to the continuance of the species, and the cells which subserve the life of the individual. And the early separation of reproductive cells from somatic cells, is alleged on the ground that this primary division of labor is that which arises between elements devoted to species-life and elements devoted to individual life. Let us not be content with words but look at the facts.
When Milne-Edwards first used the phrase "physiological division of labor," he was obviously led to do so by perceiving the analogy between the division of labor in a society, as described by political economists, and the division of labor in an organism. Every one who reads has been familiarized with the first as illustrated in the early stages, when men were warriors while the cultivation and drudgery were done by slaves and women; and as illustrated in the later stages, when not only are agriculture and manufactures carried on by separate classes, but agriculture is carried on by landlords, farmers, and laborers, while manufactures, multitudinous in their kinds, severally involve the actions of capitalists, overseers, workers, etc., and while the great function of distribution is carried on by wholesale and retail dealers in different commodities. Meanwhile students of biology, led by Milne-Edwards's phrase, have come to recognize a parallel arrangement in a living creature; shown, primarily, in the devoting of the outer parts to the general business of obtaining food and escaping from enemies, while the inner parts are devoted to the utilization of food and supporting themselves and the outer parts; and shown, secondarily, by the subdivision of these great functions into those of various limbs and senses in the one case, and in the other case into those of organs for digestion, respiration, circulation, excretion, etc. But now let us ask what is the essential nature of this division of labor. In both cases it is an exchange of services—an arrangement under which, while one part devotes itself to one kind of action and yields benefit to all the rest, all the rest, jointly and severally performing their special actions, yield benefits to it in exchange. Otherwise described, it is a system of mutual dependence: A depends for its welfare upon B, C, and D; B upon A, C, and D, and so with the rest: all depend upon each and each upon all. Now let us apply this true conception of the division of labor to that which Prof. Weismann calls a division of labor. Where is the exchange of services between somatic cells and reproductive cells? There is none. The somatic cells render great services to the reproductive cells, by furnishing them with materials for growth and multiplication; but the reproductive cells render no services at all to the somatic cells. If we look for the mutual dependence we look in vain. We find entire dependence on the one side and none on the other. Between the parts devoted to individual life and the part devoted to species-life, there is no division of labor whatever. The individual works for the species; but the species works not for the individual. Whether at the stage when the species is represented by reproductive cells, or at the stage when it is represented by eggs, or at the stage when it is represented by young, the parent does everything for it, and it does nothing for the parent. The essential part of the conception is gone: there is no giving and receiving, no exchange, no mutuality.
But now suppose we pass over this fallacious interpretation, and grant Prof. Weismann his fundamental assumption and his fundamental corollary. Suppose we grant that because the primary division of labor is that between somatic cells and reproductive cells, these two groups are the first to be differentiated. Having granted this corollary, let us compare it with the facts. As the alleged primary division of labor is universal, so the alleged primary differentiation should be universal too. Let us see whether it is so. Already, in the paragraph from which I have quoted above, a crack in the doctrine is admitted: it is said that "this differentiation was not at first absolute, and indeed it is not always so to-day." And then, on turning to page 7-4, we find that the crack has become a chasm. Of the reproductive cells it is stated that—"In Vertebrata they do not become distinct from the other cells of the body until the embryo is completely formed." That is to say, in this large and most important division of the animal kingdom, the implied universal law does not hold. Much more than this is confessed. Lower down the page we read—"There may be in fact cases in which such separation does not take place until after the animal is completely formed, and others, as I believe that I have shown, in which it first arises one or more generations later, viz., in the buds produced by the parent."
So that in other great divisions of the animal kingdom the alleged law is broken; as among the Cœlenterata by the Hydrozoa, as among the Mollusca by the Ascidians, and as among the Annuloida by the Trematode worms.
Even in ordinary life, a man whose supposition proves to be flatly contradicted by observation, is expected to hesitate; though, unhappily, he very often does not. But in the world of science, one who finds his hypothesis at variance with large parts of the evidence, forthwith abandons it. Not so Prof. Weismann. If he does not say with the speculative Frenchman, "tant pis pour les faits," he practically says something equivalent:—Propound your hypothesis; compare it with the facts; and if the facts do not agree with it, then assume potential fulfillment where you see no actual fulfillment. For this is what he does. Following his admission above quoted, concerning the Vertebrata, come certain sentences which I partially italicize:
And a little lower down the page we meet with the lines:
That is to say, it is "quite conceivable" that after sexless Cercariæ have gone on multiplying by internal gemmation for generations, the "two kinds of substance" have, notwithstanding innumerable cell-divisions, preserved their respective natures, and finally separate in such ways as to produce reproductive cells. Here Prof. Weismann does not, as in a case before noted, assume something which it is "easy to imagine," but he assumes something which it is difficult to imagine; and apparently thinks that a scientific conclusion may be thereupon safely based.
But now to what end are we asked to make a gratuitous "supposition," to accept as true something strange which is "quite conceivable," and to strain our imaginations without the slightest aid from the evidence? Simply to save Prof. Weismann's hypothesis—to shelter it against a great body of adverse facts. When we have recognized the truth that what he regards as a primary division of labor is no division of labor at all—when we see that the corollary he draws respecting the implied primary differentiation of reproductive cells from somatic cells is consequently without warrant; we have no occasion to feel troubled that his deductive conclusion is inductively disproved. We are not dismayed on finding that throughout vast groups of organisms there is shown no such antithesis as his theory requires. And we need not do violence to our thoughts in explaining away the contradictions.
Associated with the assertion that the primary division of labor is between the somatic cells and the reproductive cells, and associated with the corollary that the primary differentiation is that which arises between them, there goes another corollary. It is alleged that there exists a fundamental distinction of nature between these two classes of cells. They are described as respectively mortal and immortal, in the sense that those of the one class are limited in their powers of multiplication, while those of the other class are unlimited. And it is contended that this is due to inherent unlikeness of nature.
Before inquiring into the truth of this proposition, I may fitly remark upon a preliminary proposition set down by Prof. Weismann. Referring to the hypothesis that death depends "upon causes which lie in the nature of life itself," he says:
This last sentence has a teleological sound which would be appropriate did it come from a theologian, but which seems strange as coming from a man of science. Assuming, however, that the implication was not intended, I go on to remark that Prof. Weismann has apparently overlooked a universal law of evolution—not organic only, but inorganic and superorganic—which implies the necessity of death. The changes of every aggregate, no matter of what kind, inevitably end in a state of equilibrium. Suns and planets die, as well as organisms. The process of integration, which constitutes the fundamental trait of all evolution, continues until it has brought about a state which negatives further alterations, molar or molecular—a state of balance among the forces of the aggregate and the forces which oppose them. In so far, therefore, as Prof. Weismann's conclusions imply the non-necessity of death, they can not be sustained.
But now let us consider the above-described antithesis between the immortal Protozoa and the mortal Metazoa. An essential part of the theory is that the Protozoa can go on dividing and subdividing without limit, so long as the fit external conditions are maintained. But what is the evidence for this? Even by Prof. Weismann's own admission there is no proof. On page 285 he says:
But this is an admission which seems to be entirely ignored when there is alleged the contrast between the immortal Protozoa and the mortal Metazoa. Following Prof. Weismann's method, it would be "easy to imagine" that occasional conjugation is in all cases essential; and this easily imagined conclusion might fitly be used to bar out his own. Indeed, considering how commonly conjugation is observed, it may be held difficult to imagine that it can in any cases be dispensed with. Apart from imaginations of either kind, however, here is an acknowledgment that the immortality of Protozoa is not proved; that the allegation has no better basis than the failure to observe cessation of fission; and that thus one term of the above antithesis is not a fact, it is only an assumption.
But now what about the other term of the antithesis—the alleged inherent mortality of the somatic cells? This we shall, I think, find is no more defensible than the other. Such plausibility as it possesses disappears when, instead of contemplating the vast assemblage of familiar cases which animals present, we contemplate certain less familiar and unfamiliar cases. By these we are shown that the usual ending of multiplication among somatic cells is due not to an intrinsic cause, but to extrinsic causes. Let us, however, first look at Prof. Weismann's own statements:
"The above-mentioned considerations show us that the degree of reproductive activity present in the tissues is regulated by internal causes while the natural death of an organism is the termination—the hereditary limitation—of the process of cell-division, which began in the segmentation of the ovum" (p. 80).
Now though in the above extracts there is mention of "internal causes "determining" the degree of reproductive activity" of tissue cells, and though, on page 28, the "causes of the loss" of the power of unlimited cell-production "must be sought outside the organism, that is to say, in the external conditions of life"; yet the doctrine is that somatic cells have become constitutionally unfitted for continued cell-multiplication.
Examination will soon disclose good reasons for denying this inherent restriction. We will look at the various causes which affect their multiplication and usually put a stop to increase after a certain point is reached.
There is first the amount of vital capital given by the parent; partly in the shape of a more or less developed structure, and partly in the shape of bequeathed nutriment. Where this vital capital is small, and the young creature, forthwith obliged to carry on physiological business for itself, has to expend effort in obtaining materials for daily consumption as well as for growth, a rigid restraint is put on that cell-multiplication required for a large size. Clearly the young elephant, starting with a big and well-organized body, and supplied gratis with milk during early stages of growth, can begin physiological business on his own account on a great scale; and by its large transactions his system is enabled to supply nutriment to its multiplying somatic cells until they have formed a vast aggregate—an aggregate such as it is impossible for a young mouse to reach, obliged as it is to begin physiological business in a small way. Then there is the character of the food in respect of its digestibility and its nutritiveness. Here, that which the creature takes in requires much grinding-up, or, when duly prepared, contains but a small amount of available matter in comparison with the matter that has to be thrown away; while there, the prey seized is almost pure nutriment, and requires but little trituration. Hence, in some cases, an unprofitable physiological business, and in other cases a profitable one; resulting in small or large supplies to the multiplying somatic cells. Further, there has to be noted the grade of visceral development, which, if low, yields only crude nutriment slowly distributed, but which, if high, serves by its good appliances for solution, depuration, absorption, and circulation, to yield to the multiplying somatic cells a rich and pure blood. Then we come to an all-important factor, the cost of securing food. Here large expenditure of energy in locomotion is necessitated, and there but little—here great efforts for small portions of food, and there small efforts for great portions: again resulting in physiological poverty or physiological wealth. Next, beyond the cost of nervo-muscular activities in foraging, there is the cost of maintaining bodily heat. So much heat implies so much consumed nutriment, and the loss by radiation or conduction, which has perpetually to be made good, varies according to many circumstances—climate, medium (as air or water), covering, size of body (small cooling relatively faster than large); and in proportion to the cost of maintaining heat is the abstraction from the supplies for cell-formation. Finally, there are three all important co-operative factors, or rather laws of factors, the effects of which vary with the size of the animal. The first is that, while the mass of the body varies as the cubes of its dimensions (proportions being supposed constant), the absorbing surface varies as the squares of its dimensions; whence it results that, other things equal, increase of size implies relative decrease of nutrition, and therefore increased obstacles to cell-multiplication. The second is a further sequence from these laws—namely, that while the weight of the body increases as the cubes of the dimensions, the sectional areas of its muscles and bones increase as their squares; whence follows a decreasing power of resisting strains, and a relative weakness of structure. This is implied in the ability of a small animal to leap many times its own length, while a great animal, like the elephant, can not leap at all: its bones and muscles being unable to bear the stress which would be required to propel its body through the air. What increasing cost of keeping together the bodily fabric is thus entailed, we can not say; but that there is an increasing cost, which diminishes the available materials for increase of size, in beyond question, And then, in the third place, we have augmented expense of distribution of nutriment. The greater the size becomes, the more force must be exerted to send blood to the periphery; and this once more entails deduction from the cell-forming matters.
Here, then, we have nine factors, several of them involving subdivisions, which co-operate in aiding or restraining cell-multiplication. They occur in endlessly varied proportions and combinations; so that every species differs more or less from every other in respect of their effects. But in all of them the co-operation is such as eventually arrests that multiplication of cells which causes further growth; continues thereafter to entail slow decrease in cell-multiplication, accompanying decline of vital activities; and eventually brings cell-multiplication to an end. Now a recognized principle of reasoning—the Law of Parsimony—forbids the assumption of more causes than are needful for explanation of phenomena; and since, in all such living aggregates as those above supposed, the causes named inevitably bring about arrest of cellmultiplication, it is illegitimate to ascribe this arrest to some inherent property in the cells. Inadequacy of the other causes must be shown before an inherent property can be rightly assumed.
For this conclusion we find ample justification when we contemplate types of animals which lead lives that do not put such decided restraints on cell-multiplication. First let us take an instance of the extent to which (irrespective of the natures of cells as reproductive or somatic) cell-multiplication may go wHere the conditions render nutrition easy and reduce expenditure to a minimum. I refer to the case of the Aphides. Though it is early in the season (March), the hothouses at Kew have furnished a sufficient number of these to show that twelve of them weigh a grain—a larger number than would be required were they full-sized. Citing Prof. Owen, who adopts the calculations of Tougard to the effect that by agamic multiplication "a single impregnated ovum of Aphis may give rise, without fecundation, to a quintillion of Aphides," Prof. Huxley says:
And had Prof. Huxley taken the actual weight, one twelfth of a grain, the quintillion of Aphides would evidently far outweigh the whole human population of the globe: five billions of tons being the weight as brought out by my own calculation! Of course I do not cite this in proof of the extent to which cation of somatic cells, descending from a single ovum, may go; because it will be contended,-with some reason, that each of the sexless Aphides, viviparously produced, arose by fission of a cell which had descended from the original reproductive cell. I cite it merely to show that when the cell-products of a fertilized ovum are perpetually divided and subdivided into small groups distributed over an unlimited nutritive area, so that they can get materials for growth at no cost, and expend nothing appreciable in motion or maintenance of temperature, cell-production may go on without limit. For the agamic multiplication of Aphides has been shown to continue for four years, and to all appearance would be ceaseless were the temperature and supply of food continued without break. But now let us pass to analogous illustrations of cause and consequence open to no criticism of the kind just indicated. They are furnished by various kinds of Entozoa, of which take the Trematoda investing mollusks and fishes. Of one of them we read: "Gyrodadylus multiplies agamically by the development of a young Trematoda within the body, as a sort of internal bud. A second generation appears within the first, and even a third within the second, before the young Gyrodactylus is born." And the drawings of Steenstrup, in his Alternation of Generations, show us, among creatures of this group, a sexless individual, the whole interior of which is transformed into smaller sexless individuals, which severally, before or after their emergence, undergo similar transformations—a multiplication of somatic cells without any sign of reproductive cells. Under what circumstances do such modes of agamic multiplication, variously modified among parasites, occur? They occur where there is no expenditure whatever in motion or maintenance of temperature, and where nutriment surrounds the body on all sides. Other instances are furnished by groups in which, though the nutrition is not abundant, the cost of living is almost unappreciable. Among the Cœlenterata there are the Hydroid Polyps, simple and compound; and among the Mollusca we have various types of Ascidians, fixed and floating, Botryllidæ and Salpæ.
But now from these low animals, in which sexless reproduction, and continued multiplication of somatic cells, is common, and one class of which is named "zoöphytes," because its form of life simulates that of plants, let us pass to plants themselves. In these there is no expenditure in effort, there is no expenditure in maintaining temperature, and the food, some of it supplied by the earth, is the rest of it supplied by a medium which everywhere bathes the outer surface: the utilization of its contained material being effected gratis by the sun's rays. Just as was to be expected, we here find that agamogenesis may go on without end. Numerous plants and trees are propagated to an unlimited extent by cuttings and buds; and we have sundry plants which can not be otherwise propagated. The most familiar are the double roses of our gardens: these do not seed, and yet have been distributed everywhere by grafts and buds. Hothouses furnish many cases, as I learn from an authority second to none. Of "the whole host of tropical orchids, for instance, not one per cent has ever seeded, and some have been a century under cultivation." Again, we have the Acorus calamus, "that has hardly been known to seed anywhere, though it is found wild all over the north temperate hemisphere." And then there is the conspicuous and conclusive case of Eloidea Canadensis (alias Anacharis) introduced no one knows how (probably with timber), and first observed in 1847, in several places; and which, having since spread over nearly all England, now everywhere infests ponds, canals, and small slow rivers. The plant is diœcious, and only the female exists here. Beyond all question, therefore, this vast progeny of the first slip or fragment introduced, now sufficient. to cover many square miles were it put together, is constituted entirely of somatic cells; and this cell-multiplication, and consequent plant-growth, show no signs of decrease. Hence, as far as we can judge, these somatic cells are immortal in the sense given to the word by Prof. Weismann; and the evidence that they are so is immeasurably stronger than the evidence which leads him to assert immortality for the fissiparously-multiplying Protozoa. This endless multiplication of somatic cells has been going on under the eyes of numerous observers for forty odd years. What observer has watched for forty years to see whether the fissiparous multiplication of Protozoa does not cease? What observer has watched for one year, or one month, or one week?
Even were not Prof. Weismann's theory disposed of by this evidence, it might be disposed of by a critical examination of his own evidence, using his own tests. Clearly, if we are to measure relative mortalities, we must assume the conditions the same and must use the same measure. Let us do this with some appropriate animal—say Man, as the most open to observation. The mortality of the somatic cells constituting the mass of the human body is, according to Prof. Weismann, shown by the decline and final cessation of cell-multiplication in its various organs. Suppose we apply this test to all the organs: not to those only in which there continually arise bile-cells, epithelium-cells, etc., but to those also in which there arise reproductive cells. What do we find? That the multiplication of these last comes to an end long before the multiplication of the first. In a healthy woman, the cells which constitute the various active tissues of the body continue to grow and multiply for many years after germ-cells have died out. If similarly measured, then, these cells of the last class prove to be more mortal than those of the first. But Prof. Weismann uses a different measure for the two classes of cells. Passing over the illegitimacy of this proceeding, let us accept his other mode of measurement, and see what comes of it. As described by him, absence of death among the Protozoa is implied by that unceasing division and subdivision of which they are said to be capable. Fission continued without end, is the definition of the immortality he speaks of. Apply this conception to the reproductive cells in a Metazoon. That the immense majority of them do not multiply without end we have already seen: with very rare exceptions they die and disappear without result, and they cease their multiplication while the body as a whole still lives. But what of those extremely exceptional ones which, as being actually instrumental to the maintenance of the species, are alone contemplated by Prof. Weismann? Do these continue their fissiparous multiplications without end? By no means. The condition under which alone they preserve a qualified form of existence, is that, instead of one becoming two, two become one. A member of series A and a member of series B coalesce, and so lose their individualities. Now, obviously, if the immortality of a series is shown if its members divide and subdivide perpetually, then the opposite of immortality is shown when, instead of division, there is union. Each series ends, and there is initiated a new series, differing more or less from both. Thus the assertion that the reproductive cells are immortal, can be defended only by changing the conception of immortality otherwise implied.
Even apart from these last criticisms, however, we have clear disproof of the alleged inherent difference between the two classes of cells. Among animals the multiplication of somatic cells is brought to an end by sundry restraining conditions; but in various plants, where these restraining conditions are absent, the multiplication is unlimited. It may, indeed, be said that the alleged distinction should be reversed; since the fissiparous multiplication of reproductive cells is necessarily interrupted from time to time by coalescence, while that of the somatic cells may go on for a century without being interrupted.
In the essay to which this is a postscript, conclusions were drawn from the remarkable case of the horse and quagga there narrated, along with an analogous case observed among pigs. These conclusions have since been confirmed. I am much indebted to a distinguished correspondent who has drawn my attention to verifying facts furnished by the offspring of whites and negroes in the United States. Referring to information given him many years ago, he says: "It was to the effect that the children of white women by a white father had been repeatedly observed to show traces of black blood, in cases when the woman had previous connection with [i. e., a child by] a negro." At the time I received this information, an American was visiting me; and, on being appealed to, answered that in the United States there was an established belief to this effect. Not wishing, however, to depend upon hearsay, I at once wrote to America to make inquiries Prof. Cope, of Philadelphia, has written to friends in the South, but has not yet sent me the results. Prof. Marsh, the distinguished paleontologist, of Yale, New Haven, who is also collecting evidence, sends a preliminary letter in which he says: "I do not myself know of such a case, but have heard many statements that make their existence probable. One instance, in Connecticut, is vouched for so strongly by an acquaintance of mine, that I have good reason to believe it to be authentic.
That cases of the kind should not be frequently seen in the North, especially nowadays, is of course to be expected. The first of the above quotations refers to facts observed in the South during slavery days; and even then, the implied conditions were naturally very infrequent. Dr. W. J. Youmans, of New York, has, on my behalf, interviewed several medical professors, who, though they have not themselves met with instances, say that the alleged result, described above, "is generally accepted as a fact." But he gives me what I think must be regarded as authoritative testimony. It is a quotation from the standard work of Prof. Austin Flint, and runs as follows:
Dr. Youmans called on Prof. Flint, who remembered "investigating the subject at the time his larger work was written [the above is from an abridgment], and said that he had never heard the statement questioned."
Some days before I received this letter and its contained quotation, the remembrance of a remark I heard many years ago concerning dogs, led to the inquiry whether they furnished analogous evidence. It occurred to me that a friend who is frequently appointed judge of animals at agricultural shows, Mr. Fookes, of Fairfield, Pewsey, Wiltshire, might know something about the matter. A letter to him brought various confirmatory statements. From one "who had bred dogs for many years" he learned that—
After citing this testimony, Mr. Fookes goes on to give illustrations known to himself.
These farther evidences, to which Mr. Fookes has since added others, render the general conclusion incontestable. Coming from remote places, from those who have no theory to support, and who are some of them astonished by the unexpected phenomena, the agreement dissipates all doubt. In four kinds of mammals, widely divergent in their natures—man, horse, dog, and pig—we have this same seemingly anomalous kind of heredity made visible under analogous conditions. We must take it as a demonstrated fact that, during gestation, traits of constitution inherited from the father produce effects upon the constitution of the mother; and that these communicated effects are transmitted by her to subsequent offspring. We are supplied with an absolute disproof of Prof. Weismann's doctrine that the reproductive cells are independent of, and uninfluenced by, the somatic cells; and there disappears absolutely the alleged obstacle to the transmission of acquired characters.
Notwithstanding experiences showing the futility of controversy for the establishment of truth, I am tempted here to answer opponents at some length. But even could the editor allow me the needful space, I should be compelled both by lack of time and by ill health to be brief. I must content myself with noticing a few points which most nearly concern me.Referring to my argument respecting tactual discriminativeness, Mr. Wallace thinks that I—
Here Mr. Wallace assumes it to be self-evident that skin-sensitiveness is due to natural selection, and assumes that this must be admitted by me. He supposes it is only the unequal distribution of skin-discriminativeness which I contend is not thus accounted for. But I deny that either the general sensitiveness or the special sensitiveness results from natural selection; and I have years ago justified the first disbelief, as I have recently the second. In The Factors of Organic Evolution, pp. 66-70, I have given various reasons for inferring that the genesis of the nervous system can not be due to survival of the fittest; but that it is due to the direct effects of converse between the surface and the environment; and that thus only is to be explained the strange fact that the nervous centers are originally superficial, and migrate inward during development. These conclusions I have, in the essay Mr. Wallace criticises, upheld by the evidence which blind boys and skilled compositors furnish; proving, as this does, that increased nervous development is peripherally initiated. Mr. Wallace's belief that skin-sensitiveness arose by natural selection is unsupported by a single fact. He assumes that it must have been so produced because it is all-important to self-preservation. My belief that it is directly initiated by converse with the environment is supported by facts; and I have given proof that the assigned cause is now in operation. Am I called upon to abandon my own supported belief and accept Mr. Wallace's unsupported belief? I think not. Referring to my argument concerning blind cave animals, Prof. Lankester, in Nature of February 3, 1893, writes:
It seems to me that a supposition is here made to do duty as a fact; and that I might with equal propriety say that "possibly, or even probably," the vertebrate eye is physiologically cheap: its optical part, constituting nearly its whole bulk, consisting of a low order of tissue. There is, indeed, strong reason for considering it physiologically cheap. If any one remembers how relatively enormous are the eyes of a fish just out of the egg—a pair of eyes with a body and head attached; and if he then remembers that every egg contains material for such a pair of eyes; he will see that eye-material constitutes a very considerable part of the fish's roe; and that, since the female fish provides this quantity every year, it can not be expensive. My argument against Weismann is strengthened rather than weakened by contemplation of these facts.
Prof. Lankester asks my attention to a hypothesis of his own, published in the Encyclopædia Britannica, concerning the production of blind cave-animals. He thinks it can—
First of all, I demur to the words "many animals." Under the abnormal conditions of domestication, congenitally defective eyes may be not very uncommon; but their occurrence under natural conditions is, I fancy, extremely rare. Supposing, however, that in a shoal of young fish, there occur some with eyes seriously defective. What will happen? Vision is all-important to the young fish, both for obtaining food and for escaping from enemies. This is implied by the immense development of eyes just referred to. Considering that out of the enormous number of young fish hatched with perfect eyes, not one in a hundred reaches maturity, what chance of surviving would there be for those with imperfect eyes? Inevitably they would be starved or be snapped up. Hence the chances that a matured or partially matured semi-blind fish, or rather two such, male and female, would be swept into a cave and left behind are extremely remote. Still more remote must the chances be in the case of crayfish. Sheltering themselves as these do under stones, in crevices, and in burrows which they make in the banks, and able quickly to anchor themselves to weeds or sticks by their claws, it seems scarcely supposable that any of them could be carried into a cave by a flood. What, then, is the probability that there will be two nearly blind ones, and that these will be thus carried? Then after this first extreme improbability, there comes a second, which we may, I think, rather call an impossibility. How would it be possible for creatures subject to so violent a change of habitat to
I survive? Surely death would quickly follow the subjection to such utterly unlike conditions and modes of life. The existence of these blind cave-animals can be accounted for only by supposing that their remote ancestors began making excursions into the cave, and, finding it profitable, extended them, generation after generation, further in: undergoing the required adaptations little by little.
I turn now to Dr. Romanes. He says that I do not understand Weismann; and that the cause of degeneration to which he gives the name of "Panmixia" is not the continued selection of the smaller variations. Let us see what are Weismann's words,
"Those fluctuations on either side of the average which we call myopia and hypermetropia, occur in the same manner, and are due to the same causes, as those which operate in producing degeneration in the eyes of cave-dwelling animals" (lb., p. 89).
Here, then, are two propositions: (1) "Fluctuations on either side of the average" "operate in producing degeneration in the eyes of cave-dwelling animals." (2) "A rudimentary organ" is removed "by the operation of natural selection." Why are "fluctuations on either side of the average" named, unless it is that natural selection takes advantage of them by preserving the smaller variations? If this is not meant the use of the expression is meaningless. Yet Dr. Romanes agrees with Weismann in regarding the "degenerated eye of the Proteus as a good example of the disappearance of a complex and useless structure by Panmixia." So that Panmixia is clearly identified with the selection of the smaller variations; and for the reason that economy of nutrition is so achieved. Where, then, is the misunderstanding? That my interpretation is correct I have further reason for holding; namely, that it is the one given by Weismann's adherent. Prof. Lankester, in Nature, March 27, 1890 (pp. 487, 488). But while I can not admit my failure to understand Weismann, I confess that I do not understand Dr. Romanes. How, when natural selection, direct or reversed, is set aside, the mere cessation of selection should cause decrease of an organ irrespective of the direct effects of disuse, I am unable to see. Clearer conceptions of these matters would be reached if, instead of thinking in abstract terms, the physiological processes concerned were brought into the foreground. Beyond the production of changes in the sizes of parts by the selection of fortuitously arising variations, I can see but one other cause for the production of them—the competition among the parts for nutriment. This has the effect that active parts are well supplied and grow, while inactive parts are ill supplied and dwindle. This competition is the cause of "economy of growth"; this is the cause of decrease from disuse; and this is the only conceivable cause of that decrease which Dr. Romanes contends follows the cessation of selection. The three things are aspects of the same thing. And now, before leaving this question, let me remark on the strange proposition which has to be defended by those who deny the dwindling of organs from disuse. Their proposition amounts to this:—that for a hundred generations an inactive organ may be partially denuded of blood all through life, and yet in the hundredth generation will be produced of just the same size as in the first I
There is one other passage in Dr. Romanes' criticism—that concerning the influence of a previous sire on progeny—which calls for comment. He sets down what he supposes Weismann will say in response to my argument. "First, he may question the fact." Well, after the additional evidence given above, I think he is not likely to do that; unless, indeed, it be that along with readiness to base conclusions on things "it is easy to imagine" there goes reluctance to accept testimony which it is difficult to doubt. Second, he is supposed to reply that "the germplasm of the first sire has in some way or another become partly commingled with that of the immature ova"; and Dr. Romanes goes on to describe how there may be millions of spermatozoa and "thousands of millions" "of their contained" ids "around the ovaries, to which these secondary effects are due. But, on the one hand, he does not explain why in such case each subsequent ovum, as it becomes matured, is not fertilized by the sperm-cells present, or their contained germ-plasm, rendering all subsequent fecundations needless; and, on the other hand, he does not explain why, if this does not happen, the potency of this remaining germ-plasm is nevertheless such as to affect not only the next succeeding offspring, but all subsequent offspring. The irreconcilability of these two implications would, I think, sufficiently dispose of the supposition, even had we not daily multitudinous proofs that the surface of a mammalian ovarium is not a spermatheca. The third difficulty Dr. Romanes urges is the inconceivability of the process by which the germ-plasm of a preceding male parent affects the constitution of the female and her subsequent offspring. In response, I have to ask why he piles up a mountain of difficulties based on the assumption that Mr. Darwin's explanation of heredity by "Pangenesis" is the only available explanation preceding that of Weismann? and why he presents these difficulties to me more especially, deliberately ignoring my own hypothesis of physiological units? It can not be that he is ignorant of this hypothesis, since the work in which it is variously set forth (Principles of Biology, §§ 66-97) is one with which he is well acquainted: witness his Scientific Evidences of Organic Evolution; and he has had recent reminders of it in Weismann's Germ-plasm, where it is repeatedly referred to. Why, then, does he assume that I abandon my own hypothesis and ado[)t that of Darwin, thereby entangling myself in difficulties which my own hypothesis avoids? If, as I have argued, the germ-plasm consists of substantially similar units (having only those minute differences expressive of individual and ancestral differences of structure), none of the complicated requirements which Dr. Romanes emphasizes exist, and the alleged inconceivability disappears.
Here I must end: not intending to say more, unless for some very urgent reason, and leaving others to carry on the discussion. I have, indeed, been led to suspend for a short time my proper work only by consciousness of the transcendent importance of the question at issue. As I have before contended, a right answer to the question whether acquired characters are or are not inherited, underlies right beliefs not only in Biology and Psychology, but also in Education, Ethics, and Politics.—Contemporary Review.
- A postscript to the essay on The Inadequacy of "Natural Selection."
- See First Principles, part ii, chap, xxii, Equilibration.
- Principles of Biology, § 46 (No. 8, April, 1863).
- Ibid. This must not be understood as implying that while the mass increases as the cubes, the quantity of motion which can be generated increases only as the squares; for this would not be true. The quantity of motion is obviously measured, not by the sectioned areas of the muscles alone, but by these multiplied into their lengths, and therefore increases as the cubes. But this admission leaves untouched the conclusion that the ability to bear stress increases only as the squares, and thus limits the ability to generate motion, by relative incoherence of materials.
- The Transactions of the Linnæan Society of London, vol. xxii, p. 215. The estimate of Réaumur, cited by Kirby and Spence, is still higher—"In five generations one Aphis may be the progenitor of 5,904,900,000 descendants; and it is supposed that in one year there may be twenty generations" (Introduction to Entomology, vol. i, p. 1 75).
- A Manual of the Anatomy of Invertebrated Animals, by T. H. Huxley, p. 206.
- A Text-Book of Human Physiology. By Austin Flint, M. D., LL. D. Fourth edition. New York: D. Appleton k Co., 1888, p. 797.
- Contemporary Review, April, 1893, p. 509.
- See Social Organism in Westminster Review for January, 1860; also Principles of Socilology § 247.