a physical, or quasi-physical, explanation of an organic process, when he referred the various modes of segmentation of an ovum, complete or partial, equal or unequal and so forth, to the varying amount or the varying distribution of food yolk in association with the germinal protoplasm of the egg11. But in the main, Balfour, like all the other embryologists of his day, was engrossed by the problems of phylogeny, and he expressly defined the aims of comparative embryology (as exemplified in his own textbook) as being ãtwofold: (1) to form a basis for Phylogeny. and (2) to form a basis for Organogeny or the origin and evolution of organs12.ã
It has been the great service of Roux and his fellow-workers of the school of ãEntôÙwickeôÙlungsôÙmeôÙchaôÙnik,ã and of many other students to whose work we shall refer, to try, as His tried13 to import into embryology, wherever possible, the simpler concepts of physics, to introduce along with them the method of experiment, and to refuse to be bound by the narrow limitations which such teaching as that of Hertwig would of necessity impose on the work and the thought and on the whole philosophy of the biologist.
Before we pass from this general discussion to study some of the particular phenomena of growth, let me give a single illustration, from Darwin, of a point of view which is in marked contrast to Hallerãs simple but essentially mathôÙeôÙmatôÙiôÙcal conception of Form.
There is a curious passage in the Origin of Species14, where Darwin is discussing the leading facts of embryology, and in particular Von Baerãs ãlaw of embryonic resemblance.ã Here Darwin says ãWe are so much accustomed to see a difference in {58} structure between the embryo and the adult, that we are tempted to look at this difference as in some necessary manner contingent on growth. *But there is no reason why, for instance, the wing of a bat, or the fin of a porpoise, should not have been sketched out with all their parts in proper proportion, as soon as