THE EVOLUTION OF LIVING BEINGS. lOI 



that he ascribes a much greater effect to what he calls 

 indirect agencies: 



„I may add, judging from the vast number of new 

 varieties of plants which have been produced in the 

 „same districts and under nearly the same routine of 

 ..culture, that probably the indirect effects of domes- 

 „tication in making the organisation plastic is a much 

 „more efficient source of variation than any direct 

 „effect which external causes may have on the colour, 

 „texture or form of each part". 



Now what may this indirect effect of domestication be ? 



To answer this we must first ask: is the fact that we 

 s e e « larger number of different forms belonging to the 

 same Linneon under domestication than in nature, proof 

 that there exist more such forms under domestication 

 than in nature ? 



This of course need not be the case, it is also pos- 

 sible that forms are visible under domestication which, 

 although existing in nature, remain there hidden 

 to us. 



Suppose this were the case, what then would cause 

 their appearance under domestication? 



The answer is: isolation. 



We have seen that every heterozygote isolated, un- 

 dergoes segregation by which the recessives it con- 

 tains in a crjTptomerous way, become visible. 



Now this is exactly what domestication does : it iso- 

 lates individuals, and this simple fact explains how 

 savages, of whom no great „breeding" quahties can be 

 expected, yet succeed in raising different races of do- 

 mesticated animals and plants. 



