86 THE RATE OF GROWTH [ch. 



problems of phylogeny, and he expressly defined the aims of com- 

 parative embryology (as exemphfied in his own textbook) as being 

 *' twofold: (1) to form a basis for Phylogeny, and (2) to form a basis 

 for Organogeny, or the origin and evolution of organs*." 



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, 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 hmitations which such teaching as that 

 of Hertwig would of necessity impose on the work and the thought 

 and the whole philosophy of the biologist. 



Before we pass from this general discussion to study some of the 

 particular phenomena of growth, let m.e give an 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 "f, where Darwin 

 is discussing the leading facts of enibryology, and in particular 

 Von Baer's "law of embryonic resemblance." Here Darwin says: 

 "We are so much accustomed to see a difference in 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 any part became visible." After 

 pointing out various exceptions, with his habitual care, Darwin 

 proceeds to lay down two general principles, viz. "that shght 

 variations generally appear at a not very early period of Hfe," and 

 secondly, that "at whatever age a variation first appears in the 

 parent, it tends to reappear at a corresponding age in the offspring." 

 He then argues that it is with nature as with the fancier, who does 

 not care what his pigeons look Uke in the embryo so long as the 

 full-grown bird possesses the desired quahties : and that the process 

 of selection takes place when the birds or other animals are nearly 



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



t Ist 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 misinterpreta- 

 tion when one attempts to epitomise Darwin's carefully condensed arguments. 



