Physical Basis of Heredity and Evolution 35 



is to discover whether these possibilities can be formulated 

 into a law. 



In this connection, the phenomenon of parthenogenesis 

 should be referred to. By definition, it means the pro- 

 duction of a new individual by an unfertilized egg, and it is 

 a very common phenomenon among plants. When it is 

 remembered that ordinary cells can produce new individ- 

 uals by vegetative multiplication, and that spores can 

 reproduce, it should not be thought strange that so well- 

 nourished a cell as the egg can do the same thing. Among 

 the higher plants, however, in which the whole mechanism 

 has been worked out in greater detail, there is a significant 

 fact connected with the cases of parthenogenesis. In every 

 case investigated, the reduction in connection with spore 

 formation has not occurred, and therefore the unfertilized egg 

 contains the doubled number of chromosomes, just as though 

 it had been fertilized. In parthenogenesis, therefore, the 

 indications are that the fact to be explained is not reproduc- 

 tion by an unfertilized egg, but the failure of reduction. 



The whole history of sexual reproduction among plants 

 indicates that its primary significance is not reproduction, 

 for probably many more individuals are produced by vege- 

 tative multiplication and by spores than by the sex act. 

 This would mean that the sexual method is chiefly con- 

 cerned with other results, which are secured in connection 

 with reproduction. These results seem to be the continual 

 securing of new combinations, and new combinations cer- 

 tainly make for evolutionary progress. 



