Now it is evident, without comment, of how much value aggregates of characters ought to be in classification, if the ultimate meaning of classification be that of tracing lines of pedigree; whereas, if this ultimate meaning were that of tracing divine ideals manifested in special creation, we can see no reason why single characters are not such sure tokens of a natural arrangement as are aggregates of characters, even though the latter be in every other respect unimportant. For, on the special creation theory, we cannot explain why an assemblage, say of four or five trifling characters, should have been chosen to mark some unity of plan, rather than some one character of functional importance, which would have served at least equally well any such hypothetical purpose. On the other hand, as Darwin remarks, “we care not how trifling a character may be—let it be the mere inflection of the angle of the jaw, the manner in which an insect’s wing is folded, whether the skin be covered with hair or feathers—if it prevail throughout many and different species, especially those having very different habits of life, it assumes high value; for we can account for its presence in so many forms, with such different habits, only by inheritance from a common parent. We may err in this respect in regard to single points of structure, but when several characters, let them be ever so trifling, concur throughout a large group of beings having different habits, we may feel almost sure, on the theory of descent, that these characters have been inherited from a common ancestor; and we know that such aggregated characters have especial value in classification[2].”
It is true that even a single character, if found common to a large number of forms, while uniformly absent from others, is also regarded by naturalists as of importance for purposes of classification, although they recognise it as of a value subordinate to that of aggregates of characters. But this also is what we should expect on the theory of descent. If even any one structure be found to run through a number of animals presenting different habits of life, the readiest explanation of the fact is to be found in the theory of descent; but this does not hinder that if several such characters always occur together, the inference of genetic relationship is correspondingly confirmed. And the fact that before this inference was ever drawn, naturalists recognised the value of single characters in proportion to their constancy, and the yet higher value of aggregates of characters in proportion to their number—this fact shows that in their work of classification naturalists empirically observed the effects of a cause which we have now discovered, to wit, hereditary transmission of characters through ever-widening groups of changing species.
There is another argument which appears to tell strongly in favour of the theory of descent. We have just seen that non-adaptive structures, not being required to change in response to change of habits or conditions of life, are allowed to persist unchanged through many generations, and thus furnish exceptionally good guides in the science of classification—or, according to our theory, in the work of tracing lines of pedigree. But now, the converse of this statement holds equally true. For it often happens that adaptive structures are required to change in different lines of descent in analogous ways, in order to meet analogous needs; and, when such is the case, the structures concerned have to assume more or less close resemblances to one another, even though they have severally descended from quite different ancestors. The paddles of a whale, for instance, most strikingly resemble the fins of a fish as to their outward form and movements; yet, on the theory of descent, they must be held to have had a widely different parentage. Now, in all such cases where there is thus what is called an analogous (or adaptive) resemblance, as distinguished from what is called an homologous (or anatomical) resemblance—in all such cases it is observable that the similarities do not extend further into the structure of the parts than it is necessary that they should extend, in order that the structures should both perform the same functions. The whole anatomy of the paddles of a whale is quite unlike that of the fins of a fish—being, in fact, that of the fore-limb of a mammal. The change, therefore, which the fore-limb has here undergone to suit it to the aquatic habits of this mammal, is no greater than was required for that purpose: the change has not extended to any one feature of anatomical significance. This, of course, is what we should expect on the theory of descent with modification of ancestral characters; but on the theory of special creation it is not intelligible why there should always be so marked a distinction between resemblances as analogical or adaptive, and resemblances as homological or of meaning in reference to a natural classification. To take another and more detailed instance, the Tasmanian wolf is an animal separated from true wolves in a natural system of classification. Yet its jaws and teeth bear a strong general resemblance to those of all the dog tribe, although there are differences of anatomical detail. In particular, while the dogs all have on each side of the upper jaw four pre-molars and two molars, the Tasmanian wolf has three pre-molars and four molars. Now there is no reason, so far as their common function of dealing with flesh is concerned, why the teeth of the Tasmanian wolf should not have resembled homologically as well as analogically the teeth of a true wolf; and therefore we cannot assign any intelligible reason why, if all the species of the dog genus were separately created with one pattern of teeth, the unallied Tasmanian wolf should have been furnished with what is practically the same pattern from a functional point of view, while differing from a structural point of view. But, of course, on the theory of descent with modification, we can well understand why similarities of habit should have led to similarities of structural appearance of an adaptive kind in different lines of descent, without there being any trace of such real or anatomical similarities as could possibly point to genetic relationship.
Lastly, to adduce the only remaining argument from classification which I regard as of any considerable weight, naturalists have found it necessary, while constructing their natural classifications, to set great store on what Mr. Darwin calls “chains of affinities.” Thus, for instance, “nothing can be easier than to define a number of characters common to all birds; but with crustaceans any such definition has hitherto been found impossible. There are crustaceans at the opposite ends of the series, which have hardly a character in common; yet the species at both ends, from being plainly allied to others, and these to others, and so onwards, can be recognised as unequivocally belonging to this, and to no other class of the articulata[3].” Now it is evident that this progressive modification of specific types—where it cannot be said that the continuity of resemblance is anywhere broken, and yet terminates in modification so great that but for the connecting links no one could divine a natural relationship between the extreme members of the series,—it is evident that such chains of affinity speak most strongly in favour of a transmutation of the species concerned, while it is impossible to suggest any explanation of the fact in terms of the rival theory. For if all the links of such a chain were separately forged by as many acts of special creation, we can see no reason why B should resemble A, C resemble B, and so on, but with ever slight though accumulating differences, until there is no resemblance at all between A and Z.
I hope enough has now been said to show that all the general principles and particular facts appertaining to the natural classification of plants and animals, are precisely what they ought to be according to the theory of genetic descent; while no one of them is such as might be—and, indeed, used to be—expected upon the theory of special creation. Therefore, the only possible way in which all this uniform body of direct evidence can be met by a supporter of the latter theory, is by falling back upon the argument from ignorance. We do not know, it may be said, what hidden reasons there may have been for following all these general principles in the separate creation of specific types. Now, it is evident that this is a form of argument which admits of being brought against all the actual—and even all the possible—lines of evidence in favour of evolution. Therefore I deem it desirable thus early in our proceedings to place this argument from ignorance on its proper logical footing.
If there were any independent evidence in favour of special creation as a fact, then indeed the argument from ignorance might be fairly used against any sceptical cavils regarding the method. In this way, for example, Bishop Butler made a legitimate use of the argument from ignorance when he urged that it is no reasonable objection against a revelation, otherwise accredited, to show that it has been rendered in a form, or after a method, which we should not have antecedently expected. But he could not have legitimately employed this argument, except on the supposition that he had some independent evidence in favour of the revelation; for, in the absence of any such independent evidence, appeal to the argument from ignorance would have become a mere begging of the question, by simply assuming that a revelation had been made. And thus it is in the present case. A man, of course, may quite legitimately say, Assuming that the theory of special creation is true, it is not for us to anticipate the form or method of the process. But where the question is as to whether or not the theory is true, it becomes a mere begging of this question to take refuge in the argument from ignorance, or to represent in effect that there is no question to be discussed. And if, when the form or method is investigated, it be found everywhere charged with evidence in favour of the theory of descent, the case becomes the same as that of a supposed revelation, which has been discredited by finding that all available evidence points to a natural growth. In short, the argument from ignorance is in any case available only as a negative foil against destructive criticism: in no case has it any positive value, or value of a constructive kind. Therefore, if a theory on any subject is destitute of positive evidence, while some alternative theory is in possession of such evidence, the argument from ignorance can be of no logical use to the former, even though it maybe of such use to the latter. For it is only the possession of positive evidence which can furnish a logical justification of the argument from ignorance: in the absence of such evidence, even the negative value of the argument disappears, and it then implies nothing more than the gratuitous assumption of a theory.
I will now sum up the various considerations which have occupied us during the present chapter.
First of all we must take note that the classification of plants and animals in groups subordinate to groups is not merely arbitrary, or undertaken only for a matter of convenience and nomenclature—such, for instance, as the classification of stars in constellations. On the contrary, the classification of a naturalist differs from that of an astronomer, in that the objects which he has to classify present structural resemblances and structural differences in numberless degrees; and it is the object of his classification to present a tabular statement of these facts. Now, long before the theory of evolution was entertained, naturalists became fully aware that these facts of structural resemblances running through groups subordinate to groups were really facts of nature, and not merely poetic imaginations of the mind. No one could dissect a number of fishes without perceiving that they were all constructed on one anatomical pattern, which differed considerably from the equally uniform pattern on which all mammals were constructed, even although some mammals bore an extraordinary resemblance to fish in external form and habits of life. And similarly with all the smaller divisions of the animal and vegetable kingdoms. Everywhere investigation revealed the bonds of close structural resemblances between species of the same genus, resemblance less close between genera of the same family, resemblance still less close between families of the same order, resemblance yet more remote between orders of the same class, and resemblance only in fundamental features between classes of the same sub-kingdom, beyond which limit all anatomical resemblance was found to disappear—the different sub-kingdoms being formed on wholly different patterns. Furthermore, in tracing all these grades of structural relationship, naturalists were slowly led to recognise that the form which a natural classification must eventually assume would be that of a tree, wherein the constituent branches would display a progressive advance of organization from below upwards.