LAW OF VARIATION. 43 



which has not in some country drooping ears, and the 

 view suggested by some authors, that the drooping is 

 due to the disuse of the muscles of the ear, from the 

 animals not being much alarmed by danger, seems 

 probable." 



Climate, food and habit are the principal causes of 

 variation which are known to be in any marked degree 

 under the control of man ; and the effect of these is, 

 doubtless, in some measure indirect and subservient to 

 other laws, of reproduction, growth and inheritance, of 

 which we have at present very imperfect knowledge. 

 This is shown by the fact that the young of the same 

 litter sometimes differ considerably from each other, 

 though both the young and their parents have appa- 

 rently been exposed to exactly the same conditions of 

 life ; for had the action of these conditions been spe- 

 cific or direct and independent of other laws, if any of 

 the young had varied, the whole would probably have 

 varied in the same manner. 



Numberless hypotheses have been started to account 

 for variation. Some hold that it is as much the func- 

 tion of the reproductive system to produce individual 

 differences as it is to make the child like the parents. 

 Darwin says "the reproductive system is eminently 

 susceptible to changes in the conditions of life ; and to 

 this system being functionally disturbed in the parents 



