CHAP. I.] CIKCUMSTANCES FAVOURABLE TO SELECTION. 29 



vary less than others, yet the rarity or absence of distinct breeds of 

 the cat, the donkey, peacock, goose, &c., 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 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 organisation, though it has varied to 

 a slight extent, as I have elsewhere described. 



Some authors have maintained that the amount of variation in 

 our domestic productions is soon reached, and can never afterwards 

 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 improved in many ways within a recent 

 period ; and this implies variation. It would be equally rash to 

 assert that characters now increased to their usual 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 to be overcome, the weight of 

 body to be carried, and the power of contraction in the muscular 

 fibres. But what concerns us is that the domestic varieties of the 

 same species differ from each other in almost every character, which 

 man has attended to and selected, more than do the distinct species 

 of the same genera. Isidore Geoffroy St. Hilaire has proved this 

 in regard to size, and so it is with colour and probably with the 

 length of hair. With respect to fleetness, which depends on many 

 bodily characters, Eclipse was far fleeter, and a dray-horse is in- 

 comparably stronger than any two natural species belonging to 

 the same genus. So with plants, the seeds of the different varie- 

 ties of the bean or maize probably differ more in size, than do the 

 seeds of the distinct species in any one genus in the same two 

 families. The same remark holds good in regard to the fruit of 

 the several varieties of the plum, and still more strongly with the 

 melon, as well as in many other analogous cases. 



To sum up on the origin of our domestic races of animals and 

 plants. Changed conditions of life are of the highest importance 

 in causing variability, both by acting directly on the organisation, 

 and indirectly by affecting the reproductive system. It is not 

 probable that variability is an inherent and necessary contingent, 



