20 THE FACTORS OF ORGANIC EVOLUTION. 



&quot; 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 variation, if strongly pronounced, usually recurs only after long 

 intervals of time, any particular variation would generally be lost by 

 crossing, reversion, and the accidental destruction of the varying individuals, 

 unless carefully preserved by man.&quot; Vol. ii, 292. 



Remembering that mankind, subject as they are to this 

 domestication 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 selection alone. We cannot 

 suppose that minor traits, exemplified among others by the 

 aesthetic perceptions, can have been evolved by natural 

 selection. But if there is inheritance of functionally- 

 produced modifications of structure, evolution of such minor 

 traits is no longer inexplicable. 



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 : 



&quot;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.&quot; (Animals and Plants under 

 Domestication, vol. ii, 270.) 



