INTRODUCTION' 



II 



We have thus arrived at the form in which the problems of heredity 

 and development confront the investigator of the present day. It 

 remains to point out more clearly how they are related to the general 

 problems of evolution and to those post-Darwinian discussions in 

 which Weismann has taken so active a part. All theories of evolu- 



B 



C 



D 



F 



G ^ ^ 



Fig 4. — Cleavage of the ovum of the sea-urchin Toxopncustes, X 33°. ^''oni ''f^^- ^''^^ suc- 

 cessive divisions up to the i6-cell stage (//) occupy about two hours. / is a section of the embryo 

 (blastula) of three hours, consisting of approximately 128 cells surrounding a central cavity or 

 blastocoel. 



tion take the facts of variation and heredity as fundamental postulates, 

 for it is by variation that new characters arise and by heredity that 

 they are perpetuated. Darwin recognized two kinds of variation, 

 both of which, being inherited and maintained through the conserving 

 action of natural selection, might give rise to a permanent transfor- 

 mation of species. The first of these includes congenital or mborn 



