240 CAUSES OF VARIABILITY. Chap. XXII 



reflect on the individual differences between organic beings in 

 a state of nature, as shown by every wild animal knowing its 

 mate ; and when we reflect Dn 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 reproduction. 



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, and is not the result of what we 

 blindly call accident. The following striking case has been 

 communicated to me by Dr. William Ogle. 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 of the second dentition, on the right side 

 in the upper jaw was misplaced ; for, instead of standing in a 

 line with the others, it grew from the roof of the mouth 

 behind the first bicuspid. Neither the parents nor any other 

 members of the family were known to have exhibited any 

 similar peculiarity ; but a son of one of these girls had the 

 same tooth similarly misplaced. Now, as both the girls were 

 affected in exactly the same manner, 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 given crooked 

 fingers and misplaced bicuspid teeth to a hundred children. 

 It is of course possible that this case may have been due to 

 reversion to some long-forgotten progenitor, and this would 

 much weaken the value of the argument. I. have been led to 

 think of the probability of reversion, from having been told by 

 Mr. Galton of another case of twin girls born with their little 

 fingers slightly crocked, which they inherited from their 

 maternal grandmother. 



We will now consider the general arguments, which appear 

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

 tions of all kinds and degrees are directly or indirectly caused 



