Chap. LI CIRCUMSTANCES FAVOURABLE TO SELECTION. 31 



especially from no pleasure having been felt in the display of distinct 

 breeds ; but the goose, under the conditions to which it is exposed 

 v/hen domesticated, seems to have a singularly inflexible organisa- 

 tion, 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 utmost limit, could 

 not, after remaining fixed for many centuries, again vaiy 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, an 

 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 eveiy character, which 

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

 of the same genera. Isidore Geoffrey 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 fife 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 rever- 

 sion 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- 



