THE FACTORS OF ORGANIC EVOLUTION. 767 



of any one by the natural selection of a variation is necessarily diflB- 

 cult. Here it is : 



"Finally, as indefinite and almost illimitable variability is the usual result of 

 domestication and cultivation, with the same part or organ varying in different 

 individuals in different or even in directly opposite ways ; and as the same varia- 

 tion, if strongly pronounced, usually recurs only after long intervals of time, 

 any particular variation would generally he lost by crossing, reversion, and the 

 accidental destruction of the varying individuals, unless carefully preserved by 

 man."— Vol. ii, 292. 



Remembering that mankind, subject as they are to this domesti- 

 cation and cultivation, are not, like domesticated animals, under an 

 agency which picks out and preserves particular variations ; it results 

 that there must usually be among them, under the influence of natural 

 selection alone, a continual disappearance of any useful variations of 

 particular faculties which may arise. Only in cases of variations 

 which are specially preservative, as, for example, great cunning during 

 a relatively barbarous state, can we expect increase from natural se- 

 lection alone. "VYe cannot suppose that minor traits, exemplified among 

 others by the sesthetic perceptions, can have been evolved by natural 

 selection. But if there is inheritance of functionally-produced modi- 

 fications of structure, evolution of such minor traits is no longer inex- 

 plicable. 



Two remarks made by Mr. Darwin have implications from which 

 the same general conclusion must, I think, be drawn. Speaking of the 

 variability of animals and plants under domestication, he says : 



"Changes of any kind in the conditions of life, even extremely slight 

 changes, often suffice to cause variability. . . . Animals and plants continue to 

 be variable for an immense period after their first domestication ; ... In the 

 course of time they can be habituated to certain changes, so as to become less 

 variable; . . . There is good evidence that the power of changed conditions 

 accumulates; so that two, three, or more generations must be exposed to new 

 conditions before any effect is visible. . . . Some variations are induced by the 

 direct action of the surrounding conditions on the whole organization, or on 

 certain parts alone, and other variations are induced indirectly through the 

 reproductive system being affected in the same manner as is so common with 

 organic beings when removed from their natural conditions." — {Animals and 

 Plants under Domestication, vol. ii, 270.) 



There are to be recognized two modes of this effect produced by 

 changed conditions on the reproductive system, and consequently on 

 offspring. Simple arrest of development is one. But beyond the 

 variations of offspring arising from imperfectly-developed reproduc- 

 tive systems in parents — variations which must be ordinarily in the 

 nature of imperfections — there are others due to a changed balance of 

 functions caused by changed conditions. The fact noted by 'Mr. Dar- 

 win in the above passage, that "the power of changed conditions ac- 



