THE EVOLUTION OF LIVING BEINGS. 103 



almost unconsciously been done in all efforts to tame 

 wild animals or to cultivate wild plants. 



Domestication, even without in- 

 troducing a new source for the pro- 

 duction of new forms, consequently 

 allows us to gain forms, not or but 

 very rarely met with in nature, by 

 the mere isolation of heterozygotes. 



But domestication has not acted in this way only ; it 

 certainly has introduced new sources for the produc- 

 tion of new forms by crossing. 



It is a very curious fact, that this evident source of 

 „ variability" under domestication has always been 

 explained away. 



Almost all writers on domestication of animals or 

 plants had to acknowledge that much pointed towards 

 a multiple origin of our domesticated races by crossing, 

 and yet almost all have, notwithstanding this evi- 

 dence, pleaded for a single origin by variation from one 

 ancestral form. 



We all know, that Darwin ascribed the origin of the 

 domesticated as well as of the wild new forms to some 

 sort of heriditary variation. For this conception it is 

 evidently necessary, to show or at least to make plau- 

 sible, that the domestic races, as well as the wild varie- 

 ties", can be considered to belong to one species. 



But if we examine the evidence for this contention, 

 we at once perceive how meagre it is. 



In support of this, I will quote from Darwins Varia- 

 tion of animals and plants under domestication second 

 edition revised. London John Murray 1893. 



