114 THE EVOLUTION OF LIVING BEINGS. 



We are thus justified to conclude: 



Domestication spells segregation, followed by selec- 

 tion and isolation of the desirable segregates. 



Crossing is always at the bottom of it, this may have 

 taken place already before the animal or plant was 

 domesticated in nature; in most cases however will 

 have taken place after domesticion. 



All breeders of animals and plants know this and 

 continuously obtain novelties by crossing. 



The introduction of new forms from distant coun- 

 tries is therefore diligently resorted to, always with 

 the view of crossing them with stock already in hand, 

 in the hope to obtain novelties. 



On the other hand, there is not the slightest proof, 

 that the mere change of conditions, following upon do- 

 mestication, itself causes variability "; in all known 

 cases such variability" was the result of a cross. 



That it seems occasionally, as if spontaneous varia- 

 bility occurs in breeds, is caused by the fact that hard- 

 ly any breed is homozygotic in all its individuals, so 

 that aberrant types are born, whenever two hetero- 

 zygotes happen to mate. This f. i. is the reason that 

 from time to time red variants" occur in the dutch 

 white and black cattle. 



