150 EVOLUTION 



faction, and the key is man's accumulative 

 selection. Nature gives successive variations; 

 man adds them up, making for himself useful 

 breeds. Skilful breeders speak of the organiza- 

 tion as plastic and under control, and have 

 effected great changes within our own genera- 

 tion. Unconscious selection, which results 

 from every one trying to possess and breed 

 the best individuals, is even more important. 

 The accumulation of change which man 

 effects explains why we so often cannot 

 recognize the wild parent stocks of our culti- 

 vated plants, while its absence in countries 

 inhabited by uncivilized man explains why 

 these never yield plants worth immediate 

 culture. Man's power of selection is facili- 

 tated by keeping large numbers, in which 

 variations are more likely to occur. Facility 

 in preventing crosses is also of importance, 

 e.g. in the case of pigeons as contrasted with 

 cats; some species are, however, less variable 

 than others, e. g. the goose. 



VARIATION UNDEK NATURAL CONDITIONS. 

 Individual differences arise even in the off- 

 spring of the same parents and tend to be 

 inherited; hence they afford material for 

 natural selection to act on and accumulate, 

 precisely as they would for human selection. 

 (It may be that genera with large numbers of 



