CHAPTER XX
THE CLASSICAL TRADITION IN MODERN MORPHOLOGY
To write a history of contemporary movements from a purely objective standpoint is well recognised to be an impossible task. It is difficult for those in the stream to see where the current is carrying them: the tendencies of the present will only become clear some twenty years in the future.
I propose, therefore, in this concluding chapter to deal only with certain characteristics of modern work on the problems of form which seem to me to be derived directly from the older classical tradition of Cuvier and von Baer.
The present time is essentially one of transition. Complete uncertainty reigns as to the main principles of biology. Many of us think that the materialistic and simplicist method has proved a complete failure, and that the time has come to strike out on entirely different lines. Just in what direction the new biology will grow out is hard to see at present, so many divergent beginnings have been made—the materialistic vitalism of Driesch, the profound intuitionalism of Bergson, the psychological biology of Delpino, Francé, Pauly, A. Wagner and W. Mackenzie. But if any of these are destined to give the future direction to biology, they will in a measure only be bringing biology back to its pre-materialistic tradition, the tradition of Aristotle, Cuvier, von Baer and J. Müller. It may well be that the intransigent materialism of the 19th century is merely an episode, an aberration rather, in the history of biology—an aberration brought about by the over-rapid development of a materialistic and luxurious civilisation, in which man's material means have outrun his mental and moral growth.
Two movements seem significant in the morphology of the last decade or so of the 19th century—first, the experimental study of form, and second, the criticism of the concepts or prejudices of evolutionary morphology.
The period was characterised also by the great interest taken in cytology, following upon the pioneer work of Hertwig, van Beneden and others on the behaviour of the nuclei in fertilisation and maturation.[519] This line of work gained added importance in connection with contemporary research and speculation on the nature of hereditary transmission, and it has in quite recent years received an additional stimulus from the re-discovery of Mendelian inheritance. Its importance, however, seems to lie rather in its possible relation to the problems of heredity than in any meaning it may have for the problems of form. More significant is the revolt against the cell-theory started by Sedgwick[520] and Whitman,[521] on the ground that the organism is something more than an aggregation of discrete, self-centred cells.
The experimental work on the causes of the production and restoration of form infused new life into morphology. It opened men's eyes to the fact that the developing organism is very much a living, active, responsive thing, quite capable of relinquishing at need the beaten track of normal development which its ancestors have followed for countless generations, in order to meet emergencies with an immediate and purposive reaction. It was cases of this kind, cases of active regulation in development and regeneration, that led men like G. Wolff and H. Driesch to cast off the bonds of dogmatic Darwinism and declare boldly for vitalism and teleology.
There was the famous case of the regeneration of the lens in Amphibia from the edge of the iris—an entirely novel mode of origin, not occurring in ontogeny. The fact seems to have been discovered first by Colucci in 1891, and independently by G. Wolff in 1895.[522] The experiment was later repeated and confirmed by Fischel and other workers. Wolff drew from this and other facts the conclusion that the organism possesses a faculty of "primary purposiveness" which cannot have arisen through natural selection.[523] And, as is well known, Driesch derived one of his most powerful arguments in favour of vitalism from the extraordinary regenerative processes shown by Tubularia and Clavellina in the course of which the organism actually demolishes and rebuilds a part or the whole of its structure. But under the influence of physiologists like Loeb many workers held fast to materialistic methods and conceptions.
The great variety of regulative response of which the organism showed itself capable made it very difficult for the morphologist to uphold the generalisations which he had drawn from the facts of normal undisturbed development. The germ-layer theory was found inadequate to the new facts, and many reverted to the older criterion of homology based on destiny rather than origin. The trend of opinion was to reject the ontogenetic criterion of homology, and to refuse any morphological or phylogenetic value to the germ-layers.[524]