250 THE GENESIS OF SPECIES. [CHAP. 



Mr. Darwin also says : " There is reason to believe that 

 insects are affected by confinement like the higher animals," 

 and he gives examples. 12 



Again, he gives examples of change of plumage in the 

 linnet, bunting, oriole, and other birds, and of the tempo- 

 rary modification of the horns of a male deer during a 

 voyage. 18 



Finally, he adds that these changes cannot be attributed 

 to loss of health or vigor, " when we reflect how healthy, 

 long-lived, and vigorous many animals are under captivity, 

 such as parrots, and hawks when used for hawking, chetahs 

 when used for hunting, and elephants. The reproductive 

 organs themselves are not diseased ; and the diseases from 

 which animals in menageries usually perish, are not those 

 which in any way affect their fertility. No domestic ani- 

 mal is more subject to disease than the sheep, yet it is 

 remarkably prolific. ... It would appear that any change 

 in the habits of life, whatever these habits may be, if great 

 enough, tends to affect in an inexplicable manner the pow- 

 ers of reproduction." 



Such, then, is the singular sensitiveness of the genera- 

 tive system. 



As to the means by which that system is affected, we 

 see that a variety of conditions affect it ; but as to the 

 modes in which they act upon it, we have as yet little if any 

 clew. 



We have also seen the singular effects (in tailed Lepi- 

 doptera, etc.) of causes connected with geographical distri- 

 bution, the mode of action of which is as yet quite inexpli- 

 cable; and we have also seen the innate tendency which 

 there appears to be in certain groups (birds of paradise, etc.) 

 to develop peculiarities of a special kind. 



It is, to say the least, probable that other influences 

 exist, terrestrial and cosmical, as yet unnoted. The grad- 



12 "Animals and Plants under Domestication," vol. ii., p. 157. 



13 Ibid., p. 158. 



