Ill] OF PHYSICS AND EMBRYOLOGY 57 



Roux remarks, the task of investigating a physical mechanism in 

 embryology, — "der Ziel das Wirken zu erforschen," — has no 

 existence at all. For Balfour also, as for Hertwig, the mechanical 

 or physical aspect of organic development had httle or no attraction. 

 In one notable instance, Balfour himself adduced 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 egg*. But in the main, Balfour, hke all the 

 other embryologists of his day, was engrossed by the problems of 

 phylogeny, and he expressly defined the aims of comparative 

 embryology (as exemphfied in his own textbook) as being "two- 

 fold: (1) to form a basis for Phylogeny. and (2) to form a basis 

 for Organogeny or the origin and evolution of organsf." 



It has been the great service of Roux and his fellow-workers 

 of the school of "Entwickelungsmechanik," and of many other 

 students to whose work we shall refer, to try, as His tried J, 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 

 Haller's simple but essentially mathematical conception of Form. 



There is a curious passage in the Origin of Species^, where 

 Darwin is discussing the leading facts of embryology, and in 

 particular Von Baer's "law of embryonic resemblance." Here 

 Darwin says "We are so much accustomed to see a difference in 



* Cf. Roux, Gesammelte Ahhandlungen, ii, p. 31, 1895. 



t Treatise on Comparative Embryology, i, p. 4, 1881. 



% Cf. Fick, Anal. Anzeiger, xxv, p. 190, 1904. 



§ 1st ed. p. 444; 6th ed. p. 390. The student should not fail to consult the 

 passage in question; for there is always a risk of misunderstanding or mis- 

 interpretation when one attempts to epitomise Darwin's carefully condensed 

 arguments. 



