THE EVOLUTION OF LIVING BEINGS. 113 



In the face of this fact and of the knowledge, we have 

 obtained, that a transmittable influence of external 

 circumstances has never been proved to exist, as little 

 as any other form of hereditable variability, it is safe 

 to say that: 



Our domesticated plants and animals are the results of 

 isolation of heterozygotes, caught in nature, followed by 

 selection and isolation of the recessives, or the results of 

 crossing, followed also by segregation and selection of the 

 desirable segregates. 



Now is it possible to get definite proof for this con- 

 tention ? 



It seems to me that it is. 



If crossing is at the bottom of the origin of our do- 

 mestic races, such races must have originated as he- 

 terozygotes ; and as such heterozygotes frequently must 

 have had desirable qualities already, it stands to rea- 

 son, that in cases, in which such heterozygotes could be 

 multiplied asexually, e. g. by budding, by grafting, by 

 cuttings, by bulbs etc. one will have resorted to one of 

 these means. If therefore, crossing is the final cause of the 

 origin of new domestic races, it is reasonable to expect, 

 that a great majority of the different kinds of trees 

 and flowers, multiplied habitually in an asexual way, 

 as f. i. fruittrees and flower bulbs are, must be hete- 

 rozygotes. 



Now this is doubtless the case. 



We know that nearly all kinds of apples, hyacinths, 

 tulips etc. when sown, segregate into a great number 

 of different types — frequently in a most astonishing 

 number of them — thus proving their hybrid origin. 



