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 being aTabri 

 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 selffertilization followsacrossbetween 

 prganisms differing in one or more respects. 



If strict selffertilization 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 





