EVOLUTION IN NATURE AND UNDER 

 DOMESTICATION. 



THE facts observed in animals and plants under domestic 

 ation are of great interest for a study of the causes of variation, 

 and of the effect of selection on species. But, before we can 

 draw any far-reaching conclusions from these facts, we must 

 examine the difference between what happens in nature and 

 under cultivation. For, only if we know in how far conditions 

 are similar, and in what particulars they are dissimilar, can we 

 begin to generalize from the facts observed. 



In animals and allogamous plants, a species consists of a 

 multitude of more or less similar individuals, a "Paarungsge- 

 nossenschaft," within which matings are free and inter-crossing 

 is the rule. If nearly all the individuals have a given genotype, 

 and therefore a certain set of characters, the very few aberrant 

 individuals have no chance of propagating their type. Every 

 aberrant individual mates with a normal, and those of its off- 

 spring which are not pure for the common genotype again mate 

 with normal individuals. The very existence of a multitude of 

 genotypically identical individuals conserves the type of the 

 species. Selection within a species, natural selection, has been 

 exercising whatever influence it has for as long as the. species 

 has been living in the circumstances in which we iind it. A 

 group of animals or plants, impure for genes having a marked 

 effect on the success in life of the individuals, will probably be- 

 come pure for a certain genotype quicker than when no selec- 

 tion discriminated between individuals. If we observe varia- 

 tion within a wild species, the variability is most marked in 

 non-essential characters. 



Cross-breeding in nature, crossing between members of differ 



