50 SUMMARY OF VAEIATION. Chap. I. 



Some authors have maintained that the amount of varia- 

 tion in our domestic productions is soon reached, and can 

 never afterward 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 g'reatly improved 

 in many ways within a recent period ; and this imjilies varia- 

 tion. It would be equally rash to assert that characters now 

 increased to their vitmost limit, could not, after remaining fixed 

 for many centuries, again vary under new conditions of hfe. 

 No doul)t, 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, Mhich 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 

 color, and probably Muth the length of hair. With respect to 

 fleetness, which depends on many bodily characteristics. Eclipse 

 was far fleeter, and a drajMiorse is incomparably stronger, than 

 any two equine species. So with plants, the seeds of the dif- 

 ferent varieties of the bean or maize difl'cr more in size than 

 do the seeds of the distinct species in any one genus of the 

 same two families. The same remark holds good in regard 

 to the fruit of the several varieties c»f the plum, and still more 

 so with the melon, as well as in endless 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 im- 

 portance in causing variability, both directly l)y acting on the 

 organization, and indirectly by aft'ccting the reproductive sj's- 

 tem. 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 Avhether 

 variations shall endure. Yariability is governed by many un- 

 known laws, more especially by that of correlation. Some- 

 Ihing may be attributed to the definite action of the conditions 

 of life, but how much, we do not know. Something must be 

 :iltiil)uted to use and disuse. The final result is thus rendered 

 inllnilely comjilex. In some cases the intercrossing of abori- 

 ginally distinct species, has probably played an important part 

 in the orijxin of oin* domestic breeds. When several breeds 



