36 CIRCUMSTANCES FAVORABLE TO SELECTION. 



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 noctural rambling habits, can not 

 be easily matched, and, although so much valued by 

 women and children, we rarely see a distinct breed long 

 kept up; such breeds as we do sometimes see are almost 

 alv,ays 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 atten- 

 tion paid to their breeding; for recently in certain parts of 

 Spain and of the United States this animal has been sur- 

 prisingly modified and improved by careful selection; in 

 peacocks, from not being very easily reared and a large 

 stock not kept: in geese, from being valuable 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 singularly 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 im- 

 plies 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 re- 

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

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

 terrestrial animal, as this will be determined by the fric- 

 tion to be overcome, the weight of the body to be carried, 

 and the power of contraction in the muscular fibers. But 



