CIRCUMSTANCES FAVOURABLE TO SELECTION. 49 



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 incomparably 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 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, 

 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 



