56 ORIGIN OF SPECIES 



sert that the limit has been attained in any one case ; for al- 

 most all our animals and plants have been greatly improved in 

 many w^ays within a recent period ; and this implies variation. 

 It vi^ould be equally rash to assert that characters now in- 

 creased 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 mus- 

 cular fibres. But what concerns us is that the domestic vari- 

 eties 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. Isi- 

 dore 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 char- 

 acters, Eclipse was far fleeter, and a dray-horse is incom- 

 parably stronger than any two natural species belonging to 

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

 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 re- 

 gard to the fruit of the several varieties of the plum, and still 

 more strongly with the melon, as well as in many other anal- 

 ogous 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, under all circumstances. The greater 

 or less force of inheritance and reversion determine whether 

 variations shall endure. Variability is governed by many 

 unknown laws, of which correlated growth is probably the 

 most important. Something, but how much we do not know, 

 may be attributed to the definite action of the conditions of 

 life. Some, perhaps a great, effect may be attributed to the 

 increased use or disuse of parts. The final result is thus 



