CHAPTER 11. 



HISTORY OF THE THEORY OF HEREDITY. 



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

 lation on heredity — Evolution hypothesis of Bonnet and Ilal- 

 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 specuhitions upon he- 

 redity fail 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 hj^pothesis — 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 lieredity. 



The following list is a brief summary of wliat seem 

 to me the most important characteristics of the rejiro- 

 dtictive ]3rocess in living things: 



1. New organisms may be produced by the yarious 

 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. 



