20 Heredity. 



been produced by slow modification, tlirough the in- 

 fluence of conditions which are to a great extent open 

 to study. The attempt to trace their origin and signifi- 

 cance is not a pure specuhition, but a legitimate exercise 

 for the scientific intellect. 



As we should expect from the fascinating nature of the 

 subject, there has been no lack of speculation in the past, 

 and various h^^potheses haye been proposed from time to 

 time to account for the phenomena of heredity. These 

 hypotheses differ greatly among themselves, but they 

 may be roughly classed as epigenesis hypotheses, and evo- 

 lution hypothesis: the word evolution being here used, of 

 course, in its old sense, as contrasted with epigenesis. 



The hypothesis of evolution, pure and simj^le, as ad- 

 vocated by Bonnet and Haller, is that there is contained 

 in the agg or seed or in the male element a perfect but 

 minute organism, and that the subsequent development 

 of the egg is simply the ^'evolution" or unfolding of this 

 germ. Up to the end of the last century the jirevailing 

 ojiinion was that each ^gg contains, in a latent or dormant 

 state, a completely formed organism. The fertilization 

 of the egg was supposed to awaken this dormant germ, 

 to call its latent potential life into activity; and the pro- 

 cess of development was regarded as the unfolding and 

 growth of the already fully formed and perfect embryo. 

 The cmbr3'o ^vas held to be not produced by, but simply 

 unfolded from the ^gg, and the act of reproduction was 

 therefore regarded as eduction wot j^rodud ion. 



According to Huxley (Enc3'c. Brit., Art. Evolution) 

 " Bonnet affirms that before fecundation the hen's egg 

 contains an excessively minute but complete chick, and 

 that fecundation and incubation simply cause this germ 

 to absorb nutritious matters, which are deposited in the 

 interstices of the elementary structure of which the min» 



