There was even a tendency to concentrate attention upon the historical side of structure, upon what the animal passively inherited rather than upon what it personally achieved. Homologies were considered more interesting than analogies, vestigial organs more interesting than fœtal and larval adaptations. Convergence was anathema. The dead-weight of the past was appreciated at its full and more than its full value; and the essential vital activity of the living thing, so clearly shown in development and regeneration, was ignored or forgotten.
But evolutionary morphology for all practical purposes was a development of pure or idealistic morphology, and was powerless to bring to fruit the new conception with which evolution-theory had enriched it. The reason is not far to seek. Pure morphology is essentially a science of comparison which seeks to disentangle the unity hidden beneath the diversity of organic form. It is not immediately concerned with the causes of organic diversity—that is rather the task of the sciences of the individual, heredity and development. To take an example—the recapitulation theory may legitimately be used as a law of pure morphology, as stating the abstract relation of ontogeny to phylogeny, and the probable line of descent of any organism may be deduced from it, as a mere matter of the ideal derivation of one form from another; but an explanation of the reason for the recapitulation of ancestral history during development can clearly not be given by pure morphology unaided. From the fact that the common starfish shows in the course of its development distinct traces of a stalk[463] it is possible to infer, taking other evidence also into consideration, that the ancestors of the starfish were at one stage of their existence stalked and sessile organisms. But this leaves unanswered the question as to how and why the starfish does still repeat after so many millions of years part of the organisation of one of its remote ancestors. Why is this feature retained, and by what means has it been conserved through countless generations? It is clear that the answer can be given only by a science of the causes of the production and retention of form, by a causal morphology, based upon a study of heredity and development.
From the point of view of the pure morphologist the recapitulation theory is an instrument of research enabling him to reconstruct probable lines of descent; from the standpoint of the student of development and heredity the fact of recapitulation is a difficult problem whose solution would perhaps give the key to a true understanding of the real nature of heredity.
To make full use of the conception of the organism as an historical being it is necessary then to understand the causal nexus between ontogeny and phylogeny.
We shall see in the next chapter that the transformation of morphology from a comparative to a causal science did take place towards the end of the century, and that some progress was made towards an understanding of the relation between individual development and ancestral history, particularly by Roux and Samuel Butler, working with the fruitful Lamarckian conception of the transforming power of function.
[456] The importance of convergence came to be realised after the vogue of phylogenetic speculation had passed—see Friedmann, Die Konvergenz der Organismen, Berlin, 1904, and A. Willey, Convergence in Evolution, London, 1911. Also L. Vialleton, Elements de morphologie des Vertébrés, Paris, 1912.
[457] From this point of view there is a very profound analogy between artificial and natural selection. Upon the theory of natural selection organisms are lifeless constructs which are mechanically perfected by external agency, just as machines are improved by a process of conscious selection of the most successful among a number of competing models. (Cf. passage quoted below, on p. [308].)
[458] Arch. f. mikr. Anat., xi. (suppl.), 1874; Morph. Jahrb., ii., 1876, v. 1879, and vii., 1882.
[459] Vergleich. Anat. d. Wirbelthiere, i., pp. 200-1, 1898.
[460] For a full historical account of work on membrane and cartilage bones (as well as on the theory of the skull) see E. Gaupp, "Altere und neuere Arbeiten über den Wirbelthierschädel," Ergeb. Anat. Entw., x., 1901, and "Die Entwickelung des Kopfskelettes," in Hertwig's "Handbuch vergl. exper. Entwickelungslehre d. Wirbelthiere," iii., 2, pp. 573-874, 1905.