Form and Function: A Contribution to the History of Animal Morphology

by E. S. Russell

The sketch of his theory, written in 1842,[352] shows a very significant division into two parts—the first dealing with the positive facts of variability and the theory of natural selection, the second with the general evidence for evolution. It is in the second part that the paragraphs on morphological matters occur. In paragraph 7, on affinities and classification, Darwin points out that on the theory of evolution homological relationship would be real relationship, and the natural system would really be genealogical. In the next paragraph he notes that evolution would account for the unity of type in the great classes, for the metamorphosis of organs, and for the close resemblance which early embryos show to one another. It is of special interest to note that he definitely rejects the Meckel-Serres theory of recapitulation. "It is not true," he writes, "that one passes through the form of a lower group, though no doubt fish more nearly related to fœtal state" (p. 42). The greater divergence which adults show seems to him to be due to the fact that selection acts more on the later than on the embryonic stages. He realises very clearly how illuminative the theory of evolution is when applied to the puzzling facts of embryonic development. "The less differences of fœtus—this has obvious meaning on this view: otherwise how strange that a horse, a man, a bat should at one time of life have arteries, running in a manner which is only intelligibly useful in a fish! The natural system being on theory genealogical, we can at once see why fœtus, retaining traces of the ancestral form, is of the highest value in classification" (p. 45).

Abortive organs, too, gain significance on the evolutionary hypothesis. "The affinity of different groups, the unity of types of structure, the representative forms through which fœtus passes, the metamorphosis of organs, the abortion of others, cease to be metaphorical expressions and become intelligible facts" (p. 50).

In general, organisms can be understood only if we take into account the cardinal fact that they are historical beings. "We must look at every complicated mechanism and instinct as the summary of a long history of useful contrivances much like a work of art" (p. 51).[353]

Already in 1842 Darwin had seized upon the main principles of evolutionary morphology: the indications then given are elaborated in the thirteenth chapter of the Origin of Species (1st ed., 1859). A good part of this chapter is given up to a discussion of the principles of classification, only a few pages dealing with morphology proper. But, as Darwin rightly saw, the two things are inseparable.

We note first that there is no hint of the "scale of beings"—Darwin conceives the genealogical tree as many branched. Animals can be classed in "groups under groups," and cannot be arranged in one single series.

He discusses first what kind of characters have the greatest classificatory value. Certain empirical rules have been recognised, more or less consciously, by systematists—that analogical characters are less valuable than homological, that characters of great physiological importance are not always valuable for classificatory purposes, that rudimentary organs are often very useful, and so on. He finds that as a general rule "the less any part of the organisation is concerned with special habits, the more important it becomes for classification" (p. 414), and adduces in support Owen's remark that the generative organs afford very clear indications of affinities, since they are unlikely to be modified by special habits. These rules of classification can be explained "on the view that the natural system is founded on descent with modification; that the characters which naturalists consider as showing true affinity ... are those which have been inherited from a common parent, and, in so far, all true classification is genealogical; that community of descent is the hidden bond which naturalists have been unconsciously seeking, and not some unknown plan of creation, or the enunciation of general propositions, and the mere putting together and separating objects more or less alike" (p. 420).

In general, then, homological characters are more valuable for classificatory purposes because they have a longer pedigree than analogical characters, which represent recent acquirements of the race.

Coming to morphology proper, Darwin takes up the question of the unity of type, and the homology of parts, for which the unity of type is but a general expression.

He treats this on the same lines as E. Geoffroy St Hilaire, and Owen, referring indeed specifically to Geoffroy's law of connections. "What can be more curious," he asks, "than that the hand of a man, formed for grasping, that of a mole for digging, the leg of a horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern, and should include similar bones, in the same relative positions? Geoffroy St Hilaire has strongly insisted on the high importance of relative position or connection in homologous parts; they may differ to almost any extent in form and size, and yet remain connected together in the same invariable order" (p. 434).

The unity of plan cannot be explained on teleological grounds, as Owen has admitted in his Nature of Limbs, nor is it explicable on the hypothesis of special creation (p. 435). It can be understood only on the theory that animals are descended from one another and retain for innumerable generations the essential organisation of their ancestors. "The explanation is to a large extent simple on the theory of the selection of successive slight modifications—each modification being profitable in some way to the modified form, but often affecting by correlation other parts of the organisation. In changes of this nature, there will be little or no tendency to alter the original pattern or to transpose the parts.... If we suppose that the ancient progenitor, the archetype as it may be called, of all animals, had its limbs constructed on the existing general pattern, for whatever purpose they served, we can at once perceive the plain significance of the homologous construction of the limbs throughout the whole class" (p. 435).