THEORIES OF HEREDITY 77 



i.e., the quota of heritage from the grandparent may be 

 anything from J down to o. 



The inheritance from more remote ancestors may be 

 worked out similarly. 



IV. THEORIES OF DEVELOPMENT. 



(a) EVOLUTION VERSUS EPIGENESIS. 



We have already seen in a previous chapter that two 

 kinds of theories with regard to the development of the 

 embryo had been advanced : the one asserting that a pre- 

 formed embryo in miniature lay ready in the germ-cell to 

 unfold itself (evolve) at the propitious moment (the Evolu- 

 tion theory) ; while the rival theory of Epigenesis held that 

 the germ-cell created out of its homogeneous contents the 

 embryo anew, organ by organ. That the latter theory was 

 the correct one had been finally established by the aid of 

 microscopical observations of the actual stages of the de- 

 veloping ovum. After this it may seem strange that the 

 old controversy should have been revived in modern times. 

 Still, the problem is not the old one, as this has been finally 

 settled by the test of actual observations, but is a deeper 

 and more fundamental one. We have to deal now, not with 

 the visible formation of the embryo from cells, but with the 

 inner invisible structure of the germ- plasm. If with 

 W. Roux we define " epigenesis to be the new formation of 

 a complexity," while " evolutio means the becoming visible 

 of a pre-existing latent differentiation," the new problem 

 resolves itself into the following question : Is there a pre- 

 determined architecture of the germ-plasm, according to 

 which the embryo evolves (the new Evolutio) ? Or is there 

 no such architecture, the development of the new organism 

 being due, not to the intrinsic quality of the germ-plasm, 

 but to external causes (the new Epigenesis) ? The former 

 view, as we already know, is championed by Weismann, 

 while the latter theory has its chief defender in Oscar 

 Hertwig. Weismann maintains that a differential or quali- 



