British Quarterly Review, American Edition, Vol. LIV / July and October, 1871

by Various

It certainly would be an argument of the very highest importance, and indeed most convincing, if it could be shown that, in their minute structure, the corresponding tissues of man and animals very closely agreed. Mr. Darwin affirms that this is indeed the case, and says that the correspondence in minute structure is so close, especially in the case of man and the anthropomorphous apes, that it is impossible to exaggerate it. But strange to say, he adduces no evidence whatever in support of the assertion, although he does not hesitate to make use of the assumed close correspondence as if it had been demonstrated in the most unequivocal manner. Mr. Darwin is unquestionably correct in attaching the very highest importance to this part of the evidence. As the question of correspondence in the minute structure of tissues between man and animals has scarcely been touched upon in any of the numerous critiques which have been written upon Mr. Darwin's hypothesis, we propose to direct the reader's attention to a few details of considerable interest, affecting not only the validity of views concerning, the descent of man, but affecting also the hypothesis of evolution. It has been already stated that we are ready to admit the full force of the fact of the close correspondence if this can be proved; but, on the other hand, if constant differential characters can be distinctly demonstrated, especially in corresponding tissues of closely allied species, it must be conceded that the circumstance will be very damaging to the hypothesis of evolution; for it is very doubtful if even the very great ingenuity displayed by Mr. Darwin and his followers would enable them to offer an explanation which would be considered plausible. It is somewhat significant that the subject of minute structure, in spite of its great importance having been freely admitted, has been very lightly touched upon. So far, evolutionists have fought rather shy of the evidence to be obtained by a very minute and careful examination of the tissues; though strongly advocating careful investigations of a general character, they have been very reticent on the question of microscopic investigation, and in not a few instances there are indications of an indisposition to study minute details, as if they feared observation might be pushed too far, or too much into detail to serve their purpose. Attention is constantly directed to the general points in which different species resemble each other, and the reader becomes fully impressed with the great importance of the argument resting upon the fact of the strong similarity between man and apes, but no direct comparison in minute structure between any human and simian tissue is instituted, nor are any results of such comparisons anywhere referred to. But if, for example, it could be shown that in their minute anatomy the tissues of an ape so closely resembled those of a dog on the one hand, and of a man on the other, as that they could not be distinguished by the microscope, the fact would be of the highest importance, and would add enormously to the evidence already adduced to Mr. Darwin who lays much stress upon the close correspondence between the tissues of man and animals in minute structure, but never tells us that such comparison has been actually made by himself or by others. It is certainly remarkable that a fact which Mr. Darwin evidently considers of vast importance, and which is capable of being easily put to the test of observation, should be stated without the results of a single observation being recorded. Surely an appeal to actual experiment should have been made in at least a few instances, which would illustrate not only the close correspondence, but the absence of differences between corresponding tissues in different species. This having been done, it should then have been clearly stated in what manner this correspondence in minute structure favours the idea of the common origin of distinct species. But Mr. Darwin is content here, as in many other cases, with asserting the fact as a fact, and then stating that it helps in an important manner to establish the truth of the doctrine he advocates.

As this supposed correspondence in minute structure has never, so far as we are aware, been called in question, we shall occupy some portion of the space allotted to us in adverting to certain facts of interest, and shall supplement our observations by some remarks upon the supposed correspondence, or divergence, in chemical composition between representative solids and fluids in allied but distinct species. We must admit, with many other scientific writers, that if but a very moderate proportion of the arguments advanced by Mr. Darwin in favour of his conclusions rested upon a really firm basis of fact, the formation of species by natural selection would be established; but we have found that in many cases the arguments advanced do not bear the test of careful analysis, and some assertions crumble into dust as soon as they are exposed to investigation. We shall find reason to doubt the validity of Mr. Darwin's inferences concerning chemical composition, as well as concerning minute structure. Although undoubtedly, we do discern a general correspondence, the exceptions are so remarkable, and so far inexplicable upon Mr. Darwin's view, that we are disposed to think that the argument from it must be rejected altogether. If we study carefully the minute structure of corresponding tissues, we shall find that in many instances we are confronted with the most striking and peculiar differences, which tend to establish the idea of individuality and distinctness of origin, rather than that of the community of origin of creatures closely allied in zoological characters.

The differences in minute details in the case of creatures much alike are often very remarkable, and well worthy of attentive consideration. It may be possible to explain some of them by natural selection, but the way in which this can be done has to be pointed out. Nor is it easy to see why many individual peculiarities, that could easily be specified, should exist at all. They are certainly not required by their possessors, they do not seem either of advantage or disadvantage, and it is at least conceivable that in minute structure the tissues of all closely allied animals might exactly resemble one another. But is it not remarkable that, for instance, almost every tissue of the newt, frog, toad, and green tree-frog, has individual characteristics of its own, which could be distinguished by one who was thoroughly familiar with the microscopic characters of the textures? In many cases the differences are so wide that they could not be passed over.[64] In the newt, as would be anticipated, the elementary parts of the tissues are formed altogether upon a much larger scale than, in the other animals, and there are individual differences which are most interesting. The disciples of evolution might gain some facts in support of their theory by comparing in minute structure the tissues of the newt and proteus, in which latter animal everything is on a larger and coarser (?) scale than in the newt. But would the evolutional hypothesis gain by the application of such a test?

The nerve-fibres in every part of the body of the newt differ in many minute particulars from those of the frog, and the muscular fibres of either animal could be recognised if they were successfully prepared in precisely the same manner, so that a comparison might be instituted with fairness. But in these animals not only do corresponding tissues, exhibit peculiarities, but entire organs are totally different. The kidney of the frog diverges in so many points of structure from that of the newt, that the two organs could not be mistaken the one for the other, even if examined in the most cursory manner. Each individual tube of the newt's kidney is lined by ciliated epithelium from one end to the other, while that of the frog is so lined only at the neck. The Malpighian bodies of the two animals are different, and we believe that corresponding tissues taken from these organs could be distinguished from one another. It may be answered, 'This very instance is in favour of evolution, for the kidney tube gradually loses its ciliated lining, as we pass from the lower towards the higher batrachian form. In the latter, only the neck of the tube is ciliated, while in animals higher in the scale than the batrachia, the uriniferous tube is perfectly destitute of cilia.' Will the evolutionist be satisfied with this explanation, or will he suggest some other?

Again, if we take the skin of the four animals mentioned above—although it will be seen that there is a certain general agreement in structure to be recognised, there is not a texture of the skin which is alike in them all. The cuticle is different, the glands of the skin are differently arranged, the pigment-cells present the most marked differences; and individual characteristics are to be detected in great number by anyone who will study the subject in detail with sufficient care. We do not, however, suppose for an instant that Mr. Darwin would be unable upon his hypothesis to offer a plausible explanation of all these minute points. We are well aware that this can be done, and in a manner that to some minds may seem convincing. What we wish to press upon our readers, however, is, that so far as at this time the argument rests upon a close correspondence in minute structure, it must be given up, because the asserted close correspondence in minute structure is not based upon evidence. On the other hand, actual investigation into the structure of certain corresponding tissues demonstrates remarkable individual peculiarities, and these seem to increase in number the more thoroughly and the more minutely the tissues are explored. What if, in the case of closely allied species, such structural differences be demonstrated in every part of the body? Will the fact be urged in support of a common parentage, or in favour of some different view? It may be fairly asked, if two closely allied forms have descended from a common progenitor not far removed from either, why should almost every tissue and organ in the body exhibit individual peculiarities, not one of which can be regarded as of advantage to the creature, or as contributing in any way to its survival? The sensitive fungiform papillæ of the tongue of the common frog and of the hyla differ from one another in minute structure, and specimens could be readily distinguished. Again, it might be asked, why are the hairs of the shrew different from those of the mole, and why is the disposition of the nerve-fibres round the hair-bulb even to their minutest fibrils different in different creatures, all of which possess the particular hairs called tactile, which act as delicate organs of touch? One would have supposed that the apparatus at the side of the base of a tactile hair of a shrew would be very like that upon which the tactile hair of a mole operates, and that the mechanism in both animals would not differ much from that at the base of the tactile hairs of the mouse. But the structure of the hair is different in all three, and the arrangement of the nerves is so different that there would be no difficulty in distinguishing them from the hair-sac alone. In short, there are probably very many different forms of tactile organs, in all of which a hair is the external part, but which organs exhibit important differences of structure.

If close correspondence in minute structure is to be accepted as an argument in Mr. Darwin's favour, he will surely hardly venture to assert that differences in minute structure point to a similar conclusion, though both sets of facts might be ingeniously used in support of this eminently elastic hypothesis. If the supposed correspondence was established, the evolutionist would of course point to the fact in proof of a common parentage; but if, on the other hand, the supposed correspondence should be proved to be a fiction, he might retort triumphantly, 'Only see in what infinitely minute structural particulars the law of variation by natural selection manifests its operation!'

How are we to explain the varying form and size of the red blood-corpuscles in different animals which have been so carefully examined and measured by Mr. Gulliver? The corpuscles do not vary according to the size of the animal, nor, unless our views of classification are utterly erroneous, can any constant relation be demonstrated between the size and form of the blood-disks of the creature and its position in the zoological scale. Again, in some cases, the colourless corpuscles are much larger than the coloured ones, while in others the very reverse obtains. Moreover, in many important characters, the blood-corpuscles of animals of the same class differ remarkably. The writer of this article could multiply such facts to a great extent from the observations he has been led to make incidentally, without reference to any hypothesis whatever; but he feels almost sure that, if a series of observations were made, the distinctive characters of corresponding textures taken from closely allied animals would be enormously multiplied. Such minute anatomical investigation will doubtless be instituted, but at present the leaders of scientific thought in this country seem to consider that general observations extending over a wide range of knowledge are preferable. Mr. Darwin even supposes, or, at any rate, leads his readers to infer that he supposes, that the investigation of the structural character of man and animals has been completed, or is nearly completed. It is evident he would have us believe such to be the case, for he says that to take any view of man's origin different from his own is to admit that our own structural characteristic and those of animals are a mere snare laid to entrap our judgment—as if all our tissues and organs had been thoroughly and finally explored. We know neither our own structure nor that of any plant or animal in the world. Mr. Darwin must surely be aware that the minute anatomy of the body of man or of animals is not yet in any part fully ascertained. It is possible that, as Mr. Darwin himself has not worked much at this subject, he may have been misled by his anatomical friends; but every investigator who goes into details with due care, and with sufficient accuracy, soon finds himself compelled not only to correct the facts advanced by those who have preceded him, but is able to add to known facts many new ones. There is no reason for thinking that there is any limit to this discovery of new facts. We may go on discovering for ever, but our anatomical observations will never be complete; nor must it be supposed that, even with our present means, our present knowledge of minute structure is as far advanced as is possible.

Mr. Darwin admits in many instances the existence of certain facts which he cannot explain by his hypothesis, and in this difficulty he appeals to our 'belief in the general principle of evolution,' and suggests that, 'unless we wilfully close our eyes,' we must assent to a doctrine which he confesses is not proved by the evidence he has adduced in its support. It is, however, only by wilfully closing our eyes, and very tightly indeed, and for a long period of time, that we can hope to force the understanding to accept a belief in the 'general principles in question.'

The differences observed in the minute structure of corresponding tissues in closely allied species ought to have more closely engaged the attention of Mr. Darwin, but he is evidently quite unaware of either their extent or their number. Had he been alive to these, he would scarcely have committed himself so fully, or have left so exposed to attack his argument based on the supposition of close correspondence in structure. Structural variations in detail are indeed infinite, and it is extraordinary that Mr. Darwin's assertion of close correspondence should so long have remained unchallenged. Whatever may ultimately be accepted as the true explanation of the fact, it must be admitted that it does not support Mr. Darwin's hypothesis in its present form.

Structural difference in the tissues and organs of allied species are not, however, limited to microscopic characters. There are many broad anatomical distinctions which have never been explained, such as the absence of a part or organ in an animal very closely related to numerous other species, in every one of which not only does it exist, but is largely developed. Such cases may be regarded by the evolutionist as exceptional, and he may invent some new hypothesis to account for them. Such facts may be treated as anomalies, and referred to laws yet to be discovered, upon which correlation of growth depends. By this old method of overcoming a difficulty, facts which really tell against the favourite conclusion are made to appear to tell in its favour; but in science the exception does not prove the rule. It is clear that very much is thought of the argument from agreement in general structure between more recent forms and the ancestral forms from which they are supposed to have descended, for it has been very pointedly referred to by those who support the hypothesis of natural selection. If, however, it is proved on more minute and careful examination that, although there are some points of resemblance between species, which would render plausible the idea of a common parentage, there are also striking differences, which increase in number and importance the more they are sought for, it will be admitted that the force of this argument is much weakened; and although, after making allowance for exaggerated expression, we may admit with Mr. Huxley 'that in every single visible character man differs less from the higher apes than these do from the lower members of the same order of primates,' we are nevertheless compelled by the facts to maintain that there are so very many points in which man differs from every ape, that the argument in favour of close relationship based upon correspondence in structure completely breaks down. In fact, the differences that cannot be accounted for upon the hypothesis are more important and more numerous than the resemblances which it is advanced to explain. Of what worth is an argument resting on the fact of hundreds of representative muscles, tendons, bones, and eminences on bones, in closely allied species, if the very muscles, tendons, and bones themselves exhibit minute and constant structural differences? And if, besides these anatomical differences, we meet with differences as regards the rate of development—differences in the order of development of certain tissues and organs—differences in the structural changes going on after development is complete, what shall we infer?