CHAPTER II. , 



HISTOEY OF THE THEOEY OF HEEEDITY^ 



Requisites of a theory of heredity. Historical sketch of specu- 

 lation on heredity Evolution hypothesis of Bonnet and Hal- 

 ler Ovists and spermists Modern embryological research 

 has shown that it is impossible to accept the evolution hy- 

 pothesis in its original form Buffon's speculations upon he- 

 redity fails to account for variation Hypothesis of epigene- 

 sis This hypothesis is logically incomplete The analogy 

 between phylogeny and ontogeny gives no real explanation of 

 the properties of the ovum Haeckel's plastidule hypothesis 

 This hypothesis is not logically complete unless it involves 

 the idea of evolution Jager's hypothesis Ultimate analy- 

 sis shows that this is at bottom an evolution hypothesis No 

 hypothesis of epigenesis is satisfactory No escape from 

 some form of the evolution hypothesis This conclusion is 

 accepted by Huxley. 



1. Requisites of a theory of heredity. 



The following list is a brief summary of what seem 

 to me the most important characteristics of the repro- 

 ductive process in living things: 



1. New organisms may be produced by the various 

 forms of asexual generation and from ova. 



2. Ova may develop, in certain cases, without fertili- 

 zation. 



3. As a rule the ovum does not develop into a new or- 

 ganism until it has been fertilized, by union with a male 

 cell. 



4. The ovum and male cell will not unite unless they 

 are derived from organisms with the same or nearly the 

 same systematic affinities. 



