Chap. XXII. CAUSES OF VARIABILITY. 253 



individual differences between organic beings in a state of 

 nature, as shown by every wild animal knowing its mate ; and 

 when we reflect on the infinite diversity of the many varieties 

 of our domesticated productions, we may well be inclined to 

 exclaim, though falsely as I believe, that Variability must be 

 looked at as an ultimate fact, necessarily contingent on repro- 

 duction. 



Those authors who adopt this latter view would probably 

 deny that each separate variation has its own proper exciting 

 cause. Although we can seldom trace the precise relation 

 between cause and effect, yet the considerations presently to 

 be given lead to the conclusion that each modification must 

 have its own distinct cause. When we hear of an infant 

 born, for instance, with a crooked finger, a misplaced tooth, 

 or other slight deviation of structure, it is difficult to bring 

 the conviction home to the mind that such abnormal cases 

 are the result of fixed laws, and not of what we blindly call 

 accident. Under this point of view the following case, which 

 has been carefully examined and communicated to me by 

 Dr. William Ogle, is highly instructive. Two girls, born as 

 twins, and in all respects extremely alike, had their little fingers 

 on both hands crooked; and in both children the second 

 bicuspid tooth in the upper jaw, of the second dentition, was 

 misplaced ; for these teeth, instead of standing in a line with the 

 others, grew from the roof of the mouth behind the first 

 bicuspids. Neither the parents nor any other member of the 

 family had exhibited any similar peculiarity. Now, as both 

 these children were affected in exactly the same manner by both 

 deviations of structure, the idea of accident is at once excluded ; 

 and we are compelled to admit that there must have existed 

 some precise and sufficient cause which, if it had occurred a 

 hundred times, would have affected a hundred children. 



We will now consider the general arguments, which appear 

 to me to have great weight, in favour of the view that variations 

 of all kinds and degrees are directly or indirectly caused by the 

 conditions of life to which each being, and more especially its 

 ancestors, have been exposed. 



No one doubts that domesticated productions are more variable 

 than organic beings which have never been removed from their 



