EVOLUTION IN NATURE AND UNDER DOMESTICATION. 229 



kinds of rare species, those which are rare, because they are 

 adapted to special conditions which are not often realized, and 

 species which are rare, because they are not well adapted to 

 any environment. Species of the first group are rare locally and 

 may be common in other localities, but species of the second 

 group are rare anywhere. If natural selection within a species 

 were effective, rare species would tend to become commoner, 

 because they would become better fitted to survive. 



The inevitable result of the fact that only a fraction of the 

 individuals produced in any species can actually live on earth, 

 is a selection, a natural selection, but if the selected, surviving, 

 procreating group of individuals is genotypically identical with 

 the suppressed group, no change in type will result from the 

 selection. 



The essential differences between what happens in cultiva- 

 tion and in nature are soon apparent, if we understand, that for 

 a change in genotype, for an effect of selection, isolation of one 

 sort or another is essential. Whereas in nature, perpetual cros- 

 sing is the rule, and varieties have only a fitful existence, in 

 cultivation isolation is the rule. 



To begin with, the very act of taking a species into cultiva- 

 tion is in itself an act of isolation. The group of individuals 

 which is taken from its natural surroundings and propagated, 

 will have a total potential variability smaller than that of the 

 whole species. I believe it is very seldom that a species can be 

 taken into cultivation and propagated as such, and prove to be 

 a valuable animal or plant. The requirements under cultiva- 

 tion must necessarily be quite different from the requirement 

 in nature. The very qualities which make a plant or animal a 

 success in nature, may count against it in cultivation, and re- 

 versely, plants and animals are commonly valued for charac- 

 ters which would debar them effectively from propagating 

 without the help of man. Species which are rare in nature, may 

 happen to make more valuable cultivated plants or animals, 

 than species which have proved to be a success in nature. 



As to the effectiveness of isolation under cultivation, it must 



