72 THE ORIGIN OF SPECIES 



and can never afterwanl he exceeded. It woaH be some- 

 what 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 ut- 

 most 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 



y^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 con- 

 traction 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 Geoff roy St.-Hilaire 

 has proved this in regard to size, and so it is with color 

 and probably with the length of hair. With respect to 

 fleetness, which depends on many bodily characters, 

 Eclipse was far fleeter, and a drayhorse is incompara- 

 bly stronger, than any two natural species belonging to 

 the same genus. So with plants, the seeds of the differ- 

 ent varieties 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 ani- 

 mals and plants. Changed conditions of life are of the 



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