CIR0UM8TAN0.es fa rORABLE TO SELECTION. 37 



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 GeofEroy St. 

 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 regai'd 

 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 ani- 

 mals 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 circum- 

 stances. 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. Some- 

 thing, but how much we do not know, may be attributed 

 to the definite action of the conditions of life. Some, per- 

 haps 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 intercrossing 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 occa 

 sional 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 temporarily 

 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 



