150 EVOLUTION 



own generation. 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 cultivated 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 facilitated by keeping large num- 

 bers, 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 under Natural Conditions. 

 — Individual differences arise even in the 

 offspring 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 num- 

 bers of slightly different species — e.g. rose, 

 bramble and hawkweed — owe their protean 

 character to their variations being of no 

 service or disservice, and consequently not 

 being acted on by natural selection.) In 

 determining whether groups of similar forms 



