NATURAL SELECTION 221 



have, as is asserted, long or many horns ; pigeons with feathered feet 

 have skin between their outer toes ; pigeons with short beaks have 

 small feet, and those with long beaks large feet. Hence if man goes 

 on selecting, and thus augmenting, any peculiarity, he will almost 

 certainly modify unintentionally other parts of the structure, owing 

 to the mysterious laws of correlation. 



DARWIN'S IDEA OF THE CAUSES RESPONSIBLE FOR THE ORIGIN OF DOMESTIC RACES 



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 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 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 temporarily 

 propagated by cuttings, buds, etc., the importance of crossing 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 are of little importance to us, for their 

 endurance is only temporary. Over all these causes of Change, the 

 accumulative action of Selection, whether applied methodically and 

 quickly, or unconsciously and slowly but more efficiently, seems to 

 have been the predominant Power. 



DARWIN'S IDEA OF THE ORIGIN OF VARIETIES, SPECIES, AND GENERA IN NATURE 



Finally, varieties cannot be distinguished from species except, 

 first, by the discovery of intermediate linking forms; and, secondly, 

 by a certain indefinite amount of difference between them; for two 



