Popular Science Monthly/Volume 43/June 1893/The Inadequacy of Natural Selection III
|THE INADEQUACY OF "NATURAL SELECTION."|
By HERBERT SPENCER.
THIS very pronounced opinion will be met on the part of some by a no less pronounced demurrer, which involves a denial of possibility. It has been of late asserted, and by many believed, that inheritance of acquired characters can not occur. Weismann, they say, has shown that there is early established in the evolution of each organism, such a distinctness between those component units which carry on the individual life and those which are devoted to maintenance of the species, that changes in the one can not affect the other. We will look closely into his doctrine.
Basing his argument on the principle of the physiological division of labor, and assuming that the primary division of labor is that between such part of an organism as carries on individual life and such part as is reserved for the production of other lives, Weismann, starting with "the first multicellular organism," says that — "Hence the single group would come to be divided into two groups of cells, which may be called somatic and reproductive — the cells of the body as opposed to those which are concerned with reproduction" (Essays upon Heredity, p. 27).
Though he admits that this differentiation "was not at first absolute, and indeed is not always so to-day," yet he holds that the differentiation eventually becomes absolute in the sense that the somatic cells, or those which compose the body at large, come to have only a limited power of cell-division, instead of an unlimited power which the reproductive cells have; and also in the THE INADEQUACY OF * NATURAL SELECTION:' 163
sense that eventually there ceases to be any communication be- tween the two, further than that implied by the supplying of nutriment to the reproductive cells by the somatic cells. The out- come of this argument is that, in the absence of communication, changes induced in the somatic cells, constituting the individual, can not influence the natures of the reproductive cells, and can not therefore be transmitted to posterity. Such is the theory. Now let us look at a few facts — some familiar, some unfamiliar.
His investigations led Pasteur to the positive conclusion that the silkworm diseases are inherited. The transmission from par- ent to offspring resulted, not through any contamination of the surface of the egg by the body of the parent while being deposited, but resulted from infection of the egg itself — intrusion of the parasitic organism. Generalized observations concerning the dis- ease called pebrine enabled him to decide by inspection of the eggs which were infected and which were not: certain modifi- cations of form distinguishing the diseased ones. More than this, the infection was proved by microscopical examination of the contents of the egg ; in proof of which he quotes as follows from Dr. Carlo Vittadini :
" II resulte de mes recherches sur les graines, k I'epoque oil commence le d6- veloppement du germe, que les corpuscles, une fois apparus dans I'cbuf, augmen- tent graduellement en nombre, a mesure que I'embryon se d6veloppe; que, dans les derniers jours de I'incubation, I'oeuf en est plein, au point de faire croire que la majeure partie des granules du jaune se sont transformes en corpuscules.
" Une autre observation importante est que I'embryon aupsi est souill6 de cor- puscules, et a un degr6 tel qu'on peut soupgonner que I'infection du jaune tire son origine du germe lui-m^me; en d'autres term es que le germe est primordiale- ment infect6, et porte en lui-mfeme ces corpuscules tout comme les vers adultes, lrapp6s du mfeme mai."*
Thus, then, the substance of the egg, and even its innermost vital part, is permeable by a parasite sufficiently large to be mi- croscopically visible. It is also of course permeable by the invisi- ble molecules of protein, out of which its living tissues are formed, and by absorption of which they subsequently grow. But, accord- ing to Weismann, it is not permeable by those invisible units of protoplasm out of which the vitally active tissues of the parent are constituted : units composed, as we must assume, of variously arranged molecules of protein. So that the big thing may pass, and the little thing may pass, but the intermediate thing may not pass !
A fact of kindred nature, unhappily more familiar, may be next brought in evidence. It concerns the transmission of a dis- ease not unfrequent among those of unregulated lives. The high-
- Les Maladies des Vers i Soie, par L. Pasteur, i, 39.
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est authority concerning this disease, in its inherited form, is Mr. Jonathan Hutchinson; and the following are extracts from a letter I have received from him, and which I publish with his assent :
"I do not think that there can be any reasonable doubt that a very hirge ma- jority of those who suffer from inlierited syphilis take the taint from the male parent. ... It is the rule when a man marries who has no remaining local lesion, but in whom the taint is not eradicated, for his wife to remain apparently well, while her child may suffer. No doubt the child infects its mother's blood, but this does not usually evoke any obvious symptoms of syphilis. ... I am sure I have seen hundreds of syphilitic infants whose mothers had not, so far as I could ascertain, ever displayed a single symptom."
See, then, to what we are committed if we accept Weismann's hypothesis. We must conclude that, whereas the reproductive cell may be effectually invaded by an abnormal living element in the parental organism, those normal living elements which con- stitute the vital protoplasm of the parental organism, can not evade it. Or if it be admitted that both intrude, then the impli- cation is that, whereas the abnormal element can so modify the development as to cause changes of structure (as of the teeth), the normal element can cause no changes of structure ! *
We pass now to evidence not much known in the world at large, but widely known in the biological world, though known in so incomplete a manner as to be undervalued in it. Indeed, when I name it probably many will vent a mental pooh-pooh. The fact to which I refer is one of which record is preserved in the museum of the College of Surgeons, in the shape of paintings^ of a foal borne by a mare not quite thoroughbred, to a sire which was thoroughbred— a foal which bears the markings of the quag- ga. The history of this remarkable foal is given by the Earl of Morton, F. R. S., in a letter to the President of the Royal Society (read November 23, 1820). In it he states that wishing to domes-
��* Curiously enough, Weismann refers to, and recognizes, syphilitic infection of the re- productive cells. Dealing with Browu-S6quard's cases of inherited epilepsy (concerning which, let me say, that I do not commit myself to any derived conclusions), he says : " In the case of epilepsy, at any rate, it is easy to imagine [many of Weismann's arguments are based on things ' it is easy to imagine '] that the passage of some specific organism through the reproductive cells may take place, as in the case of syphiUs " (p. 82). Here is a sam- ple of his reasoning. It is well known that epilepsy is frequently caused by some periph- eral irritation (even by the lodging of a small foreign body under the skin), and that, among peripheral irritations causing it, imperfect healing is one. Yet though, in Brown-Sequard's cases, a peripheral irritation caused in the parent by local injury was the apparent origin, Weismann chooses gratuitously to assume that the progeny were infected by " some spe- cific organism," which produced the epilepsy ! And then, though the epileptic virus, like the syphilitic virus, makes itself at home in the egg, the parental protoplasm is not ad- mitted!
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ticate the quagga, and having obtained a male, but not a female, he made an experiment.
" I tried to breed from the male qua^ga and a young chestnut mare of seven- eighths Arabian blood, and which had never been bred from ; the result was the production of a female hybrid, now five years old, and bearing, both in her form and in her color, very decided indications of her mixed origin. I subsequently parted with the seven-eighths Arabian mare to Sir Gore Ouseley, who has bred from her by a very fine black Arabian horse. I yesterday morning examined the produce, namely, a two-year-old filly and a year-old colt. They have the charac- ter of the Arabian breed as decidedly as can be expected, where fifteen-sixteenths of the blood are Arabian ; and they are fine specimens of that breed ; but both in their color and in the hair of their manes, they have a striking resemblance to the quagga. Their color is bay, marked more or less like the qaagga in a darker tint. Both are distinguished by the dark line along the ridge of the back, the dark stripes across the fore-hand, and the dark bars across the back part of the legs,"*
Lord Morton then names sundry further correspondences. Dr. Wollaston, at that time President of the Royal Society, who had seen the animals, testified to the correctness of his description, and, as shown by his remarks, entertained no doubt about the al- leged facts. But good reason for doubt may be assigned. There naturally arises the question — How does it happen that parallel results are not observed in other cases ? If in any progeny cer- tain traits not belonging to the sire, but belonging to a sire of preceding progeny, are reproduced, how is it that such anoma- lously-inherited traits are not observed in domestic animals, and indeed in mankind ? How is it that the children of a widow by a second husband do not bear traceable resemblances of the first husband ? To these questions nothing like satisfactory replies seem forthcoming ; and, in the absence of replies, skepticism, if not disbelief, may be held reasonable.
There is an explanation, however. Forty years ago I made acquaintance with a fact which impressed me by its significant implications ; and has for this reason, I suppose, remained in my memory. It is set forth in the Journal of the Royal Agricultural Society, vol. xiv (1853), pp. 214 et seq., and concerns certain results of crossing English and French breeds of sheep. The writer of the translated paper, M. Malingid-Nouel, Director of the Agri- cultural School of La Charmoise, states that when the French breeds of sheep (in which were included " the mongrel Merinos ") were crossed with an English breed, " the lambs present the fol- lowing results. Most of them resemble the mother more than the father ; some show no trace of the father." Joining the admis- sion respecting the mongrels with the facts subsequently stated, it is tolerably clear that the cases in which the lambs bore no
- Philosophical Transactions of the Royal Society for the Year 1821, Part I, pp. 20-24.
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traces of the father were cases in which the mother was of pure breed. Speaking of the results of these crossings in the second generation "having 75 per cent of English blood," M, Nouel says : " The lambs thrive, wear a beautiful appearance, and com- plete the joy of the breeder. ... No sooner are the lambs weaned than their strength, their vigor, and their beauty begin to decay. ... At last the constitution gives way. ... he remains stunted for life," the constitution being thus proved unstable or un- adapted to the requirements. How, then, did M. Nouel succeed in obtaining a desirable combination of a fine English breed with the relatively poor French breeds ?
" He took an animal from 'flocks originally sprung from a mixture of the two distinct races that are establislied in these two provinces [Berry and La Sologne],' and these he 'united with animals of another mixed breed. . , , which blended the Tourangelle and native Merino blood of La Beauce and Touraine, and ob- tained a mixture of all four races ' without decided character, without fixity. . . . but possessing the advantage of being used to our climate and management.'
"Putting one of these 'mixed-blood ewes to a pure New-Kent ram. , . . one obtains a lamb containing fifty-hundredths of the purest and most ancient Eng- lish blood, with twelve and a half hundredths of four different French races, which are individually lost in the preponderance of English blood, and disappear almost entirely, leaving the improving type in the ascendant. ... All the lambs produced strikingly resembled each other, and even Englishmen took them for animals of their own country.' "
M. Nouel goes on to remark that when this derived breed was bred with itself, the marks of the French breeds were lost. " Some slight traces could be detected by experts, but these soon disap- peared."
Thus, we get proof that relatively pure constitutions predomi- nate in progeny over much mixed constitutions. The reason is not difificult to see. Every organism tends to become adapted to its conditions of life ; and all the structures of a species, accus- tomed through multitudinous generations to the climate, food, and various influences of its locality, are molded into harmoni- ous co-operation favorable to life in that locality: the result being that in the development of each young individual, the tendencies conspire to produce the fit organization. It is other- wise when the species is removed to a habitat of different charac- ter, or when it is of mixed breed. In the one case its organs, partially out of harmony with the requirements of its new life, become partially out of harmony with one another ; since, while one influence, say of climate, is but little changed, another influ- ence, say of food, is much changed ; and consequently, the per- turbed relations of the organs interfere with their original stable equilibrium. Still more in the other case is there a disturbance of equilibrium. In a mongrel the constitution derived from each
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source repeats itself as far as possible. Hence a conflict of tend- encies to evolve two structures more or less unlike. The tenden- cies do not harmoniously conspire ; but produce partially incon- gruous sets of organs. And evidently where the breed is one in which there are united the traits of various lines of ancestry, there results an organization so full of small incongruities of structure and action, that it has a much-diminished power of maintaining its balance ; and while it can not withstand so well adverse influences, it can not so well hold its own in the offspring. Concerning parents of pure and mixed breeds respectively, sev- erally tending to reproduce their own structures in progeny, we may, therefore, say figuratively that the house divided against itself can not withstand the house, of which the members are in concord.
Now if this is shown to be the case with breeds the purest of which have been adapted to their habitats and modes of life dur- ing some few hundred years only, what shall we say when the question is of a breed which has had a constant mode of life in the same locality for ten thousand years or more, like the quagga ? In this the stability of constitution must be such as no domestic animal can approach. Relatively stable as may have been the constitutions of Lord Morton's horses, as compared with the con- stitutions of ordinary horses, yet, since Arab horses, even in their native country, have probably in the course of successive con- quests and migrations of tribes become more or less mixed, and since they have been subject to the conditions of domestic life, differing much from the conditions of their original wild life, and since the English breed has undergone the perturbing effects of change from the climate and food of the East to the climate and food of the West, the organizations of the horse and mare in ques- tion could have had nothing like that perfect balance produced in the quagga by a hundred centuries of harmonious co-operation. Hence the result. And hence at the same time the interpretation of the fact that analogous phenomena are not perceived among domestic animals, or among ourselves ; since both have relatively mixed, and generally extremely mixed, constitutions, which, as we see in ourselves, have been made generation after generation, not by the formation of a mean between two parents, but by the jum- bling of traits of the one with traits of the other, until there exist no such conspiring tendencies among the parts as cause repetition of combined details of structure in posterity.
Expectation that skepticism might be felt respecting this al- leged anomaly presented by the quagga-marked foal, had led me to think over the matter; and I had reached this interpretation before sending to the College of Surgeons Museum (being unable to go myself) to obtain the particulars and refer to the records.
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When there was brought to me a copy of the account as set forth in the Philosophical Transactions, it was joined with the infor- mation that there existed an appended account of pigs, in which a parallel fact had been observed. To my immediate inquiry — " "Was the male a wild pig ? " — there came the reply : " I did not observe." Of course I forthwith obtained the volume, and there found what I expected. It was contained in a paper communi- cated by Dr. Wollaston from Daniel Giles, Esq., concerning his " sow and her produce," which said that
"she was one of a well-known black and white breed of Mr. Western, the Mem- ber for Essex. About ten years since I put her to a boar of the wild breed, and of a deep chestnut color, which I had just received from Hatfield House, and which was soon afterward drowned by accident. The pigs produced (which were her first litter) partook in appearance of both boar and sow, but in some the chestnut color of the boar strongly prevailed.
"The sow was afterward put to a boar of Mr. Western's breed (the wild boar having been long dead). The produce was a litter of pigs, some of which, we observed with much surprise, to be stained and clearly marked with the chestnut color which had prevailed in the former litter."
Mr. Giles adds that in a second litter of pigs, the father of which was of Mr. Western's breed, he and his bailiff believe there was a recurrence, in some, of the chestnut color, but admits that their " recollection is much less perfect than I wish it to be." He also adds that, in the course of many years' experience, he had never known the least appearance of the chestnut color in Mr. Western's breed.
What are the probabilities that these two anomalous results should have arisen, under these exceptional conditions, as a matter of chance ? Evidently the probabilities against such a coinci- dence are enormous. The testimony is in both cases so good that, even apart from the coincidence, it would be unreasonable to re- ject it; but the coincidence makes acceptance of it imperative. There is mutual verification, at the same time that there is a joint interpretation yielded of the strange phenomenon, and of its non- occurrence under ordinary circumstances.
And now, in the presence of these facts, what are we to say ? Simply that they are fatal to Weismann's hypothesis. They show that there is none of the alleged independence of the reproductive cells ; but that the two sets of cells are in close communion. They prove that while the reproductive cells multiply and arrange them- selves during the evolution of the embryo, some of their germ- plasm passes into the mass of somatic ceils constituting the parental body, and becomes a permanent component of it. Fur- ther, they necessitate the inference that this introduced germ- plasm, everywhere diffused, is some of it included in the repro- ductive cells subsequently formed. And if we thus get a demon-
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stration that the somewhat different units of a foreign germ-plasm permeating the organism, permeate also the subsequently-formed reproductive cells, and affect the structures of the individuals arising from them, the implication is that the like happens with those native units which have been made somewhat different by modified functions: there must be a tendency to inheritance of acquired characters.
One more step only has to be taken. It remains to ask what is the flaw in the assumption with which Weismann's theory sets out. If, as we see, the conclusions drawn from it do not corre- spond to the facts, then, either the reasoning is invalid, or the original postulate is untrue. Leaving aside all questions concern- ing the reasoning, it will suffice here to show the untruth of the postulate. Had his work been written during the early years of the cell-doctrine, the supposition that the multiplying cells of which the Metazoa and the Metaphyta are composed, become com- pletely separate, could not have been met by a reasonable skepti- cism ; but now, not only is skepticism justifiable, but denial is called for. Some dozen years ago it was discovered that in many cases vegetal cells are connected with one another by threads of protoplasm — threads which unite the internal protoplasm of one cell with the internal protoplasms of cells around. It is as though the pseudopodia of imprisoned rhizopods were fused with the pseudopodia of adjacent imprisoned rhizopods. We can not reason- ably suppose that the continuous network of protoplasm thus con- stituted has been produced after the cells have become adult. These protoplasmic connections must have survived the process of fission. The implication is that the cells forming the embryo- plant retained their protoplasmic connections while they multi- plied, and that such connections continued throughout all subse- quent multij)lications — an implication which has, I believe, been established by researches upon germinating palm-seeds. But now we come to a verifying series of facts which the cell-structures of animals in their early stages present. In his Monograph of the Development of Peripatus Capensis, Mr. Adam Sedgwick, F. R. S., Reader in Animal Morphology at Cambridge, writes as follows : —
" All the cells of the ovum, ectodermal as well as endodermal, are connected together by a fine protoplasmic reticulum" (p. 41).
" The continuity of the various cells of the segmenting ovum is primary, and not secondary; i.e., iu the cleavage the segments do not completely separate from one another. But are we justified in speaking of cells at all in this case? The fully segmented ovum is a syncytium^ and there are not and have not been at any stage cell limits^ (p. 41).
" It is becoming more and more clear every day that the cells composing the tissues of animals are not isolated units, but that they are connected with one
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another. I need only refer to the connection known to exist between connective- tissue cells, cartilage cells, epithelial cells, etc. And not only may the cells of one tissue bo continuous with each other, but they may also be continuous with the cells of other tissues" (pp. 47, 48).
" Finally, if the protoplasm of the body is primitively a syncytium, and the ovum until maturity a part of that syncytium, the separation of the generative products does not differ essentially from the internal gemmation of a Protozoon, and the inheritance by the offspring of peculiarities first appearing in the parent, though not explained, is rendered less mysterious; for the protoplasm of the whole body being continuous, change in the molecular constitution of any part of it would naturally be expected to spread, in time, through the whole mass " (p. 49).
Mr. Sedgwick's subsequent investigations confirm these con- clusions. In a letter of December 27, 1892, passages, -whicli lie allows me to publish, run as follows :
" All the embryological studies that I have made since that to which you refer confirm me more and more in the view that the connections between the cells of adults are not secondary connections, but primary, dating from the time when the embryo was a unicellular structure. . . . My own investigations on this sub- ject have been confined to the Arthropoda, Elasmobranchii, and Aves. I have thoroughly examined the development of at least one kind of each of these groups, and I have never been able to detect a stage in which the cells were not continu- ous with each other; and I have studied innumerable stages from the beginning of cleavage onward."
So that the alleged independence of the reproductive cells does not exist. The soma — to use Weismann's name for the aggregate of cells forming the body — is, in the words of Mr. Sedgwick, " a continuous mass of vacuolated protoplasm " ; and the reproductive cells are nothing more than portions of it separated some little time before they are required to perform their functions.
Thus the theory of Weismann is doubly disproved. Inductively we are shown that there does take place that communication of characters from the somatic cells to the reproductive cells, which he says can not take place ; and deductively we are shown that this communication is a natural sequence of connections between the two which he ignores: his various conclusions are deduced from a postulate which is untrue.
From the title of this essay, and from much of its contents, nine readers out of ten will infer that it is directed against the views of Mr. Darwin. They will be astonished on being told that, contrariwise, it is directed against the views of those who, in a considerable measure, dissent from Mr. Darwin. For the inher- itance of acquired characters, which it is now the fashion in the biological world to deny, was, by Mr. Darwin, fully recognized and often insisted on. Such of the foregoing arguments as touch Mr. Darwin's views, simply imply that the cause of evolution which at first he thought unimportant, but the importance of
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which he increasingly perceived as he grew older, is more im- portant than he admitted even at the last. The neo-Darwinists, however, do not admit this cause at all.
Let it not be supposed that this explanation implies any dis- approval of the dissentients, considered as such. Seeing how little regard for authority I have myself usually shown, it would be absurd in me to reflect in any degree upon those who have re- jected certain of Mr. Darwin's teachings, for reasons which they have thought sufficient. But while their independence of thought is to be applauded rather than blamed, it is, I think, to be re- gretted that they have not guarded themselves against a long- standing bias. It is a common trait of human nature to seek some excuse when found in the wrong. Invaded self-esteem sets up a defense, and anything is made to serve. Thus it happened that when geologists and biologists, previously holding that all kinds of organisms arose by special creations, surrendered to the battery opened upon them by The Origin of Species, they sought to minimize their irrationality by pointing to irrationality on the other side. " Well, at any rate, Lamarck was in the wrong." " It is clear that we were right in rejecting his doctrine." And so, by duly emphasizing the fact that he overlooked " Natural Selection" as the chief cause, and by showing how erroneous were some of his interpretations, they succeeded in mitigating the sense of their own error. It is true their creed was that at success- ive periods in the Earth's history, old Floras and Faunas had been abolished and others introduced ; just as though, to use Prof. Huxley's figure, the table had been now and again kicked over and a new pack of cards brought out. And it is true that La- marck, while he rejected this absurd creed, assigned for the facts reasons some of which are absurd. But in consequence of the feeling described, his defensible belief was forgotten and only his indefensible ones remembered. This one-sided estimate has become traditional ; so that there is now often shown a subdued contempt for those who suppose that there can be any truth in the conclusions of a man whose general conception was partly sense, at a time when the general conceptions of his contempo- raries were wholly nonsense. Hence results unfair treatment — hence result the different dealings with the views of Lamarck and of "Weismann.
"Where are the facts proving the inheritance of acquired characters " ? ask those who deny it. Well, in the first place, there might be asked the counter-question — Where are the facts which disprove it ? Surely if not only the general structures of organisms, but also many of the modifications arising in them, are inheritable, the natural implication is that all modifications are inheritable ; and if any say that the inheritablencss is limited
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to those arising in a certain way, tlie onus lies on tliem of prov- ing that those otherwise arising are not inheritable. Leaving this counter-question aside, however, it will suffice if we ask another counter-question. It is asserted that the dwindling of organs from disuse is due to the successive survivals in posterity of individuals in which the organs had varied in the direction of decrease. Where now are the facts supporting this assertion ? Not one has been assigned or can be assigned. Not a single case can be named in which panmixia is a proved cause of diminution. Even had the deductive argument for 'panmixia been as valid as we have found it to be invalid, there would still have been re- quired, in pursuance of scientific method, some verifying induc- tive evidence. Yet though not a shred of such evidence has been given, the doctrine is accepted with acclamation, and adopted as part of current biological theory. Articles are written and let- ters published in which it is assumed that this mere speculation, justified by not a tittle of proof, displaces large conclusions previ- ously drawn. And then, passing into the outer world, this unsup- ported belief affects opinion there too ; so that we have recently had a Right Honorable lecturer who, taking for granted its truth, represents the inheritance of acquired characters as an exploded hypothesis, and thereupon proceeds to give revised views of human affairs.
Finally, there comes the reply that there are facts proving the inheritance of acquired characters. All those assigned by Mr. Darwin, together with others such, remain outstanding when we find that the interpretation by panmixia is untenable. Indeed, even had that hypothesis been tenable, it would have been inap- plicable to these cases ; since in domestic animals, artificially fed and often overfed, the supposed advantage from economy can not be shown to tell; and since, in these cases, individuals are not naturally selected during the struggle for life in which certain traits are advantageous, but are artificially selected hj man with- out regard to such traits. Should it be urged that the assigned facts are not numerous, it may be replied that there are no per- sons whose occupations and amusements incidentally bring out such facts; and that they are probably as numerous as those which would have been available for Mr. Darwin's hypothesis, had there been no breeders and fanciers and gardeners who, in pursuit of their profits and hobbies, furnished him with evidence. It may be added that the required facts are not likely to be numerous, if biologists refuse to seek for them.
See, then, how the case stands. Natural selection, or survival of the fittest, is almost exclusively operative throughout the vege- tal world and throughout the lower animal world, characterized by relative passivity. But with the ascent to higher types of
�� � animals, its effects are in increasing degrees involved with those produced by inheritance of acquired characters; until, in animals of complex structures, inheritance of acquired characters becomes an important, if not the chief, cause of evolution. We have seen that natural selection can not work any changes in organisms save such as conduce in considerable degrees, directly or indirectly, to the multiplication of the stirp; whence failure to account for various changes ascribed to it. And we have seen that it yields no explanation of the co-adaptation of co-operative parts, even when the co-operation is relatively simple, and still less when it is complex. On the other hand, we see that if, along with the transmission of generic and specific structures, there tend to be transmitted modifications arising in a certain way, there is a strong a priori probability that there tend to be transmitted modifications arising in all ways. We have a number of facts confirming this inference, and showing that acquired characters are inherited — as large a number as can be expected, considering the difficulty of observing them and the absence of search. And then to these facts may be added the facts with which this essay set out, concerning the distribution of tactual discriminativeness. While we saw that these are inexplicable by survival of the fittest, we saw that they are clearly explicable as resulting from the inheritance of acquired characters. And here let it be added that this conclusion is conspicuously warranted by one of the methods of inductive logic, known as the method of concomitant variations. For throughout the whole series of gradations in perceptive power, we saw that the amount of the effect is proportionate to the amount of the alleged cause. — Contemporary Review.