No. 579] PROGRESSIVE EVOLUTION 



173 



matin-determined characters, for, as Dr. Gates has re- 

 cently shown in the case of (Enothera, mutation may 

 result from abnormal distribution of the chromosomes in 

 the reduction division. 7 



We have next to inquire whether or not the Mendelian 

 results are really in any way inconsistent with the gen- 

 eral theory of evolution outlined in the earlier part of 

 this address. Here we are obviously face to face with 

 the old dispute between epigenesis and preformation. 

 The theory of ontogeny which I first put forward is 

 clearly epigenetic in character, while the theory of unit 

 characters, represented in the germ-cells by separate 

 ' ' factors," is scarcely less clearly a theory of preforma- 

 tion, and of course the conception of definite organ-form- 

 ing substances in the cytoplasm falls under the same 

 category. The point which I now wish to emphasize is 

 that the ideas of epigenesis and preformation are not 

 not inconsistent with one another, and that, as a matter 

 of fact, ontogenetic development is of a dual nature, an 

 epigenesis modified by what is essentially preformation. 



We have already dealt briefly with the question of 

 organ-forming substances in the cytoplasm, and it must, 

 I think, be clear that the existence of these is in no way 

 incompatible with a fundamental epigenesis. We shall 

 find directly that the same is true of Mendelian "fac- 

 tors" or Weismannian ' ' determinants." 



We have seen that it is possible to conceive of even 

 a complex organism as inheriting nothing from its parent 

 but a minute speck of protoplasm, endowed with poten- 

 tial energy, and a sequence of suitable environments, 

 the interaction between the two bringing about a similar 

 result in each suceeding generation, with a slow progres- 

 sive evolution due to the operation of the law of accu- 

 mulation of surplus energy. If any of the conditions of 

 development are changed the result, as manifested in 

 the organization of the adult, must undergo a corre- 

 sponding modification. Suppose that the chromatin sub- 



7 Quarterly Journal of Microscopical Science, Vol. LIX, p. 557. 



