CIRCUMSTANCES FAVORABLE TO SELECTION. 35 



to be overcome, the weight of the 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 spe- 

 cies 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 Saint 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 dray-horse is comparably 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 anagolous 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 organization, 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 condi- 

 tions of life. Some, perhaps a great, effect may be attributed 

 to the increased use or disuse of parts. The final result is 

 thus rendered infinitely complex. In some cases the inter- 

 crossing of aboriginally distinct species appears to have 

 played an important part in the origin of our breeds. When 

 several breeds have once been formed in any country, their 

 occasional intercrossing, with the aid of selection, has, no 

 doubt, largely aided in the formation of new sub-breeds ; 

 but the importance of crossing has been much exaggerated, 

 both in regard to animals and to those plants which are 

 propagated by seed. With plants which are teniporarity 

 propagated by cuttings, buds, etc., the importance of cross- 

 ing is immense ; for the cultivator may here disregard the 

 extreme variability both of hybrids and of mongrels, and 

 the sterility of hybrids ; but plants not propagated by seed 



