Homology

“ Accordingly, form is one of the least important elements to be considered in determining a homology. "Nature," he wrote in one of his early papers, "tends to repeat the same organs in the same number and in the same relations, and varies to infinity only their form. In accordance with this principle I shall have to draw my conclusions, in the determining the bones of the fish's skull, not from a consideration of their form, but from a consideration of their connections." ”

“ Such are the facts of distribution in space and in time; the singular phenomena brought to light by the study of development; the structural relations of species upon which our systems of classification are founded; the great doctrines of philosophical anatomy, such as that of homology, or of the community of structural plan exhibited by large groups of species differing very widely in their habits and functions. ”

“ Be that as it may, the evolutionist interprets the resemblances of homology as surviving vestiges of an ancient ancestral type, which have managed to persist in the descendants notwithstanding the transformations wrought in the latter by the process of progressive divergence. ”

“ The conception of a morphological type, which was then supported in England by the great influence of (Sir) Richard Owen [q. v.] , may be understood from his definition of homology, which he interprets 'as signifying that essential character of a part which belongs to it in its relation to a predetermined pattern, answering to the "idea" of the archetypal world in the Platonic cosmogony, which archetype or primal pattern is the basis supporting all the modifications of such part ... in all animals possessing it' (Owen, On the Nature of Limbs, 1849) . ”

“ A moment’s reflection, however, will make it clear that, in thus confounding the definition proper with its theoretical interpretation, the modern biologist is guilty of a logical atrocity. Homology, after all, is a simple anatomical fact, which can be quite adequately defined in terms of observation ”

“ Homology in form is thus the reciprocal of likeness in kind, so that homologies fall under the same law with kind, and it may be affirmed that whatever is true of an object is true of its homologue in so far as they are identical, which is but another statement of the law already given in classification, that whatever is true of a thing is true of its class identity. We have seen that there is a vast system of homologies extending throughout the universe, commencing with perfect homology in the simple element ”

“ We shall see that they form patterns, considerable in size and of a curious variety of shape, whose boundaries can be firmly outlined, and which are little worlds in themselves. They have the unique merit of retaining all their peculiarities unchanged throughout life, and afford in consequence an incomparably surer criterion of identity than any other bodily feature. They may be made to throw welcome light on some of the most interesting biological questions of the day, such as heredity, symmetry, correlation, and the nature of genera and species. ”

“ That part of biological science which deals with form and structure is called Morphology—that which concerns itself with function, Physiology—so that we may conveniently speak of these two senses or aspects of “species”—the one as morphological, the other as physiological. Regarded from the former point of view, a species is nothing more than a kind of animal or plant, which is distinctly definable from all others, by certain constant and not merely sexual, morphological peculiarities. ”

“ Morphology then is not only a study of material things and of the forms of material things, but has its dynamical aspect, under which we deal with the interpretation, in terms of force, of the operations of Energy. And here it is well worth while to remark that, in dealing with the facts of embryology or the phenomena of inheritance, the common language of the books seems to deal too much with the material elements concerned, as the causes of development, of variation or of hereditary transmission. ”

“ Any form or condition of structure under a species which differs in its characteristics from those typical to the species, as in color, shape, size, and the like, and which is capable either of perpetuating itself for a period, or of being perpetuated by artificial means; also, any of the various forms under a species meeting the conditions mentioned. A form characterized by an abnormity of structure, or any difference from the type that is not capable of being perpetuated through two or more generations, is not called a variety. ”

“ A classification which shall represent the process of ancestral evolution is, in fact, the end which the labors of the philosophical taxonomist must keep in view. But it is an end which cannot be attained until the progress of palæontology has given us far more insight than we yet possess, into the historical facts of the case. Much of the speculative 'phylogeny,' which abounds among my present contemporaries, reminds me very forcibly of the speculative morphology, unchecked by a knowledge of development, which was rife in my youth. ”

“ The morphology is treated as external, involving the development of form in the plant kingdom, relations of symmetry, branch systems, the shoot, the root, and the ontogeny of plants; and internal, embracing the histology and anatomy In the special part the theory of evolution is credited with having first afforded a true basis for a natural system of classification, expressive of relationship and family. ”

“ This is the first scheme of a real “natural classification” of living things. From this we pass to special morphology. But this fearless sketch of the specialised genealogical tree, according to the new ideas, puts general morphology in its true light. We are made to feel that it is not all mere theory. To-morrow—nay, to-day—the whole practice of zoology and botany will have to be remodelled on the new principles. ”

“ The cellate homologies of the man are repeated in all mammals, while the same facts can be seen in birds, reptiles, batrachians and fishes, for all the pentalogic classes present a vast hierarchy of homologies, which illustrate the theme of morphology. ”

“ This analogy of forms, which with all their differences seem to have been produced according to a common original type, strengthens our suspicions of an actual relationship between them in their production from a common parent, through the gradual approximation of one animal-genus to another—from those in which the principle of purposes seems to be best authenticated, i.e., from man down to the polype, and again from this down to mosses and lichens, and finally to the lowest stage of Nature noticeable by us, viz., to crude matter. ”

“ He conceived the idea that an absolute unity of type prevails throughout organic nature as regards each set of organs. Out of this idea grew his gradually formed belief that similarity of structure might imply identity of origin—that, in short, one species of animal might have developed from another. ”

“ Each higher animal is composed of so many members and organs, of such diverse variations, that naturalists could never agree among themselves as to just where a balance of affinities between resemblances and differences should be struck; whether, for example, a given species varied so much from the type species of a genus—say the genus Gothic house—as to belong properly to an independent genus—say Romanesque house ”

“ The summum genus, again, may be viewed in relation to a given universe or suppositio (that is, any limited area of existence now the object of attention) , or to the whole universe. If we take the animal kingdom as our suppositio, Animal is the summum genus; but if we take the whole universe, 'All things' is the summum genus.
"Porphyry's tree" is used to illustrate this doctrine. It begins with a summum genus, 'Substance,' and descends by adding differences, step by step, to the infima species, 'Man.' It also illustrates Division by Dichotomy. ”

“ And so on in regard to all other things. By overlooking the differences, and attending to the resemblances of singulars, we form a species; by overlooking the differences, and attending to the resemblances of species, we form a genus; by overlooking the differences, and attending to the resemblance of genera, we form a still higher genus, until we ascend up to ens, or "Being," the highest generalisation of ordinary ontology as described in the common schoolbooks upon logic. ”

“ The present treatise, devoted to anthropogeny, is divided into two parts: the first, ontogeny, or the history of individual human organisms, concerns itself chiefly with germ history or embryology; and the second, on phylogeny, is a history of the evolution of the various animal forms, from which man has descended in the course of countless ages. Phylogeny is thus a history of evolution, and embraces the sub-sciences of paleontology and genealogy. ”

“ There is nature-knowledge and there is man-knowledge; there is scientific experience and there is vital experience. Let the reader track down this contrast into his own inmost being, and he will understand what I mean.
All modes of comprehending the world may, in the last analysis, be described as Morphology. The Morphology of the mechanical and the extended, a science which discovers and orders nature-laws and causal relations, is called Systematic. The Morphology of the organic, of history and life and all that bears the sign of direction and destiny, is called Physiognomic. ”

“ THE organic relations or homologies of structure, showing connections of different beings with one another, which, in ignorance of their real bearing, were formerly made useful only for the classification of animals and plants into natural groups, have acquired a new significance since the doctrine of evolution has been brought to bear upon questions of origin and of the progress through time and space of living beings and their relations with those beings whose former existence is revealed by paleontology. ”

“ An origin such as is here advocated will also agree with the peculiar character of the modifications of form and structure which obtain in organized beings—the many lines of divergence from a central type, the increasing efficiency and power of a particular organ through a succession of allied species, and the remarkable persistence of unimportant parts such as colour, texture of plumage and hair, form of horns or crests, through a series of species differing considerably in more essential characters. ”

“ Morphology not merely studies the development of structures, but it studies the relationships of plants which are thus revealed, and hence is interested in what is known as phylogeny, that is, the ancestral history of plant groups. ”

“ Morphology, in natural science, teaches us that the whole animal and vegetable creation is formed upon certain fundamental types and patterns, which can be traced under various modifications and transformations through all the rich variety of things apparently of most dissimilar build. ”

“ The history of the origin of a type, as species, genus, order, etc., is simply the history of the origin of the structure or structures which define those groups respectively. It is nothing more nor less than this, whether a man or an insect be the object of investigation. ”

“ Taking repose as a genus, we have as species of it, though not exactly co-ordinate with one another, tranquillity with a mental differentia (repose of mind) , rest, whether of mind or body, sleep, with the differentia of unconsciousness (privative) . Synonyms are species, or varieties, wherever any difference can be detected in them; and to discriminate them we must first find the generic meaning; for which there may, or may not, be a single word. Thus, equality, sameness, likeness, similarity, resemblance, identity, are synonyms ”

“ Now we have seen that the natural system of classification gives a true picture of the genealogical relationships of organisms, that the smaller and larger classificatory groups correspond to greater or lesser branches of the genealogical tree. If ontogeny is a recapitulation of phylogeny, we must expect to find the embryo repeating the organisation first of the ancestor of the phylum, then of the ancestor of the class, the order, the family and the genus to which it belongs. ”

“ So that there can be no doubt whatsoever that any plant and any animal may vary in nature; that varieties may arise in the way I have described,—as spontaneous varieties,—and that those varieties may be perpetuated in the same way that I have shown you spontaneous varieties are perpetuated; I say, therefore, that there can be no doubt as to the origin and perpetuation of varieties in nature. ”

“ The resemblances of structure, wherever we make the comparison between different species, are referable to an ideal plan of animal construction, followed throughout a class of animals, and adjusted to their peculiar differences which distinguish one species from another, just as in the vegetable world there is an ideal plan of construction of trees followed throughout a class of plants, and adjusted to the peculiar differences which distinguish one kind of tree from another. ”

“ Now that from which the difference constituting the species is derived, is always related to that from which the genus is derived, as actuality is related to potentiality. For animal is derived from sensitive nature, by concretion as it were, for that is animal, which has a sensitive nature. ”