34 CIRCUMSTANCES FAVORABLE TO SELECTION. 



respect enclosure of the land plays a part. Wandering 

 savages or the inhabitants of open plains rarely possess 

 more than one breed of the same species. Pigeons can be 

 mated for life, and this is a great convenience to the fancier, 

 for thus many races may be improved and kept true, though 

 mingled in the same aviary ; and this circumstance must 

 have largely favored the formation of new breeds. Pigeons, 

 I may add, can be propagated in great numbers and at a 

 very quick rate, and inferior birds may be freely rejected^ 

 as when killed they serve for food. On the other hand, 

 cats, from their nocturnal rambling habits, cannot be easily 

 matched, and, although so much valued by women and chil- 

 dren, we rarely see a distinct breed long kept up ; such breeds 

 as we do sometimes see are almost always imported from 

 some other country. Although I do not doubt that some 

 domestic animals vary less than others, yet the rarity or 

 absence of distinct breeds of the cat, the donkey, peacock, 

 goose, etc., may be attributed in main part to selection not 

 having been brought into play : in cats, from the difficulty 

 in pairing them ; in donkeys, from only a few being kept by 

 poor people, and little attention paid to their breeding ; for 

 recently in certain parts of Spain and of the United States 

 this animal has been surprisingly modified and improved by 

 careful selection ; in peacocks, from not being very easily 

 reared and a large stock not kept; in geese, from being valu- 

 able only for two purposes, food and feathers, and more 

 especially from no pleasure having been felt in the display 

 of distinct breeds ; but the goose, under the conditions to 

 which it is exposed when domesticated, seems to have a sin- 

 gularly inflexible organization, though it has varied to a 

 slight extent, as I have elsewhere described. 



Some authors have maintained that the amount of varia- 

 tion in our domestic productions is soon reached, and can 

 never afterward be exceeded. It would be somewhat rash 

 to assert that the limit has been attained in any one case; 

 for almost all our animals and plants have been greatly im- 

 proved in many ways within a recent period; and this 

 implies variation. It would be equally rash to assert that 

 characters now increased to their utmost limit, could not, 

 after remaining fixed for many centuries, again vary under 

 new conditions of life. No doubt, as Mr. Wallace has 

 remarked with much truth, a limit will be at last reached. 

 For instance, there must be a limit to the fleetness of any 

 terrestrial animal, as this will be determined by the friction 



