68 THE EVOLUTION OF LIVING BEINGS. 



stop evolution and let heredity take its course. 

 Now this is different from what happens in nature ; 

 there, no homozygous form is ever sure of beingal'abri 

 of a cross, except if strict self fertilisation reigns supre- 

 me which it but rarely, if ever, does. Consequently we 

 must look into the question what is likely to happen 

 in nature after a cross. 



Calculation of what is likely to happen if 

 strict self fertilization follows a cross bet ween 

 organisms differingin one ormorerespects. 



If strict self fertilization takes place, the progeny of 

 each homozygous individual necessarily must be ho- 

 mozygous also, so that every homozygous individual 

 starts a species. The heterozygotes on the other hand, 

 will continue to segregate in all subsequent generations, 

 and give rise to a mixture of homozygotes and hetero- 

 zygotes. 



The principles which underlie the matter are there- 

 fore, in Jenning's words, the following: 



(1) In self-fertilized organisms, all characteristics that 

 become once homozygotic, remain homozygotic 

 for ever after, since there is no method in self- 

 fertilization of introducing a gamete that is di- 

 verse in this respect. 



(2) Characteristics, heterozygotically represented be- 

 come homozygotic in a certain proportion of the 

 offspring. 



The problem becomes essentially this : in what pro- 

 portion do the heterozygotic characters become homo- 

 zygotic, and how great a proportion of all the organisms 



