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VII.

THE EVIDENCE FURNISHED BY THE SCIENCE OF LIKENESSES.

In the preceding chapter it was shown, incidentally to the subject of limbs and their nature, that science makes it a duty of the highest importance to discover and trace the resemblances which frequently exist between apparently diverse and unlike structures. Such likenesses were illustrated by a reference to the similarity which could readily be found to exist between such outwardly unlike organs as the arm of man, the wing of the bird, the fore-leg of the horse, the paddle of the whale or dolphin, and the wing of the bat. In a minor degree also, but still provable from the same standpoint, the paired fins of fishes could be shown to agree with the limbs of other animals to which they present no obvious affinities. Beneath the diverse appearances of limbs, one and the same type thus appears to exist. An examination of the hard parts, or skeletons, of these appendages, readily reveals the likeness which adaptation to diverse conditions of life has produced. In connection with the limblikenesses in question, certain important considerations connected with the meaning of such similarities were briefly noted. How, or why, a common type or plan should be discernible beneath well-nigh endless variety of outward form and function, was a question which naturally obtruded itself upon the notice of the scientific observer. Such a query, it was remarked, presented, like so many other matters of scientific interest, but two methods of solution. In the one case the reply might take the form of the unquestioning and tacit assumption that such things were so formed from the beginning according to some ideal plan, or type-for the construction of which type, however, no reason can be assigned. "Conformity to a type " is an expression which merely restates what everybody admits, and what the examination of the limbs, on any hypothesis, plainly shows. To say that things "were created so presents a complete parallel to the famous "woman's reason" in the "Two Gentlemen of Verona ;" or to Tom Brown's equally renowned explanation of the dislike to Dr. Fell-a parody, by the way, on Martial

Non amo te, Sabidi, nec possum dicere quare;

Hoc tantum possum dicere, non amo te.

Turning to the other side of the question, all that is mysterious and inexplicable on the special-creation hypothesis, becomes clear enough on that of "development" and "modification." By the idea of development is implied the derivation of the similar forms, or parts, from some common type, through natural laws of heritage and descent. By "modification," or "adaptation," we mean to indicate that potent power or factor, which, seizing the common type, moulds the structure-limb or body—to the special way of life in which the being ultimately comes to walk.

If the latter idea be correct or feasible, we can readily assign a reason why limbs, or any other series of structures in a given set of animals, should present such a close likeness. "Conformity to type" is no meaningless expression when used by the evolutionist. By his theory, he views this conformity as a proof of the bloodrelationship-far or near, as the case may be-of the animals which exhibit the likeness in question. Such similarity is a proof of affinity, which can only be accounted for, in all its bearings, on the supposition that the beings exhibiting it are really kith and kin, but of varying degrees of relationship. It can readily be understood how important in the eyes of the modern naturalist this study of likenesses has become, since the facts it reveals largely assist him in constructing the true pedigree of the living world. There are many other considerations which serve to show the important nature of such a branch of inquiry-an importance equalled only by the interest which its pursuit is certain to evoke. When, for instance, it can be found that two organs so utterly unlike as the air-bladder of a fish and the lungs of a man are in reality closely connected in their nature, the information which the study of likenesses places at our disposal is seen to be of a kind which tends very materially to extend the knowledge that Bacon declared aided "the relief of man's estate." And the task of seeking and finding resemblances has had its due effect in solving not a few of the puzzles of biology. Only from the considerations it brings to view, and through the influence of the new way in which it compels us to regard forms and organs, has the mystery of such a subject as that of "rudimentary organs been dispelled. The splint-bones of a horse, when examined by the light of this study, guide us, as we have seen, to the history of the equine race; and the transformations of animals and plants teem with new interest when investigated on the principles which the science of likenesses brings to view. It is to the details of such a subject that we now invite attention. Our illustrations will be culled from both worlds of life; and in our search after the likenesses whereon hangs the past-history of the living being, we may perchance light upon considerations not far removed from the wider questions that border the origin of man himself.

"Ho

The "science of likenesses" is known to specialists as mology," and it may further our ready appreciation of the details to be presently treated in these pages if we make mention likewise of the term "analogy" and its meaning. The latter word is, as a rule, very loosely used in ordinary life. Scientifically employed, its meaning is clear enough. In a dictionary we find it explained as meaning "correspondence, or likenesses in some ways, proportions, or effects." Obviously, the term is used in a general sense to mean any degree of likeness, resemblance, or relationship between objects. In science, the word "analogy" has but one distinct meaning. It implies identity or correspondence in function or use, and nothing more. When two things are used for the same purpose they are "analogous ;" and no further resemblances or likenesses are required in science to justify the use of the term. Every one knows that a bird's wing is a very different structure from that of a fly or butterfly. The one is really a fore-limb; the other being merely an expansion of the skin of the body. But despite their wide difference in structure, they are truly "analogous," being used for one and the same purpose, that of flight. In this sense alone, can any two objects be truly termed "analogous."

Now, turning to "homology," we discover a deeper relationship between organs and parts than that indicated by analogy. That two things may be truly named "homologous" it is not necessary to think of their use in any sense. The all-important consideration on which the science of likenesses hangs, is the fact of identity or correspondence in fundamental structure or in origin. Such a correspondence is illustrated by the subject of limbs already referred to. The arm of man, the fore-leg of a horse, and the wing of a bird, are used each for a different purpose. They are not "analogous," but they are undoubtedly "homologous," because, beneath the diversity of form and function, we can readily perceive the striking similarity of fundamental structure or type. Thus things may not be what they seem, when viewed by homology-for the wings of bird and butterfly, alike in the popular sense, are utterly unlike; and regarded in the same light many things are what they do not seem. The seeming unlikenesses of arm, wing, and fore-leg are thus merely superficial, and serve to hide the deeper realities that link them firmly together as the same in type, and presumably the same in origin. It may happen, lastly, that two organs may be both analogous and homologous. But the presence of both degrees of likenesses is at the best accidental, or induced by like conditions of life which do not affect the deeper considerations which homology brings before us. The wing of the bird and that of the bat are formed each from a fore-limb-although in diverse ways-and each subserves the purpose of flight. Analogy and homology seem to run in parallel lines in this

instance. But the conditions in virtue of which a quadruped like the bat has acquired its powers of flight may have been, and probably were, different in nature, as they certainly were in time, from those under which the bird learned to soar in the air. This latter point, however, is foreign to the main issue before us. Sufficient for our present purpose are the thoughts, that homology and analogy are two distinct things; that homology indicates the deeper and real likeness between organs and parts; and that these two forms of likeness are not necessarily connected or coexistent.

So much by way of introduction to the subject of the science of likenesses. It requires but little guidance to enable the mind to follow up the line of thought already mentioned in the preceding remarks, which shows the function of this branch of inquiry in detecting the hidden relationships and bonds which connect one living being with another, or one class of organisms with a neighbouring class. Such relationships, as every one knows, are indicated by the systems of classification and arrangement which form an important part of every science, and, one may add, of many matters connected with every-day existence as well. Thus, the classification of the objects under his study or care is equally important for botanist and librarian; and in either case the aim of the system of arrangement is to bring together things that are like, and to separate those that are unlike. It matters not how this procedure is effected. Classifications vary with well-nigh each person who undertakes their formation; and the needless multiplication of systems of arrangement, equally with the persistent invention of new cognomens for already well-named species, constitute the two chief sorrows of the well-regulated scientific mind. The best classification is of course the "natural;" but it so happens that this particular arrangement is not always easy of construction: a fact chiefly explicable on the ground that the natural relationships of living beings are often hard to seek and difficult to find. When the popular classification of the fish with the whale-one, it may be added, not characteristic of primitive minds alone-is replaced by the union of the whale with the quadrupeds-seeing that it has warm blood, brings forth its young alive, and nourishes them by means of milk-a grossly artificial system of arrangement is superseded by a true and natural one. That a whale need not be a fish because it swims, or is fish-like, is thus evident; and the correctness of our arrangement of whales and fishes, and of the whole animal and plant worlds, must of necessity depend on the completeness of our knowledge of the objects we intend to classify.

Now, it is exactly the difficulties which stand in the way of forming a natural arrangement of animals and plants which are lightened by the study of homology as the science of likenesses. From the mere arrangement and classification of living beings, it may

be readily seen how we advance through the study of scientific resemblances to questions of deeper import, connected, in these latter days, with the problem of the very beginnings and origin of all living things. Before the days of evolution-at least, as represented in its typical phases of modern times-speculative philosophy was hard at work, trying to discover the "archetype" underlying the familiar types and varied plans of animal and plant structure. Goethe and Oken, for instance, by the most remarkable of coincidences, ventilated an idea concerning the ideal plan of the skull, which had been independently suggested to each philosopher by a casual glance at the bleached skull of a sheep in the one case and of a deer in the other. This idea was expressed in the theory worked out with patience and care amongst ourselves by Professor Owen, and known as the "vertebral theory of the skull." Briefly stated, it was held that the skull in reality consisted of modified vertebræ (or joints of the backbone); and that, so far from being a something different from the other parts of the skeleton, the skull was really modelled on the type of the spine. Owen recognised four such vertebræ in the skull ; and it need hardly be remarked that the views of Owen, as expressions of philosophical anatomy, were far in advance of those of Oken and Goethe, the former of whom went so far in the matter of speculation pure and simple as to assert that in the skull the whole body was represented in miniature. The head, according to Oken, was a kind of multum in parvo of the bodily structures. Therein his subjective philosophy actually found fingers and toes in the shape of the teeth. But the history of zoology includes the recital of a hot and strong controversy over the ideas emanating from Oken and Goethe, and emended and improved by Owen. Soon Owen's views were denied and combated, amongst others by Huxley, in 1858, who held them to be disproved by the study of the skull's development. The skull from its earliest phases was maintained to exhibit a very marked difference from the spine: and if two structures thus differed in their earliest phases, and when their type should have been most apparent, how, it was asked, could their identity be insisted upon? A long and elaborate series of researches has, since the time we speak of, been undertaken with reference to the homology of the skull. And with what result, it may be asked, to the idea of real likeness or unlikeness between skull and spine? The answer to this question would vary with the scientific predilections of the person who replied. But it is not too much to assert that the impetus which was first given to the search after a likeness has been increased by the light which evolution and the science of likenesses have together thrown on the reason why not merely skull and spine should resemble each other, but why likenesses and differences-due to multifarious and varying conditions of life and development—should also exist between these structures.