Modification and Variation. 317 



structure. What is the effect on congenital variations ? 

 Whereas all the other pendulums are still damped down 

 by natural selection as before, the oscillation of the pen- 

 dulum which represents a variation in this bony structure is 

 no longer checked. It is free to swing as much as it can. 

 Congenital variations in the same direction as the adaptive 

 modification will be so much to the good of the individual 

 concerned. They will constitute a congenital predisposition 

 to that strengthening of the part which is essential for 

 survival. Variations in the opposite direction tending to 

 thwart the adaptive modification will be disadvantageous, 

 and will be eliminated. Thus, if the conditions remain 

 constant for many generations, congenital variation will 

 gradually render hereditary the same strengthening of 

 bone structure that was provisionally attained by plastic 

 modification. The effects are precisely the same as they 

 would be if the modification in question were directly 

 transmitted in a slight but cumulatively increasing degree ; 

 they are reached, however, in a manner which involves no 

 such transmission. 



To take a particular case : Let us grant that in the 

 evolution of the horse tribe it was advantageous to this line 

 of vertebrate life that the middle digits of each foot should 

 be largely developed, and the lateral digits reduced in 

 size ; and let us grant that this took its rise in adaptive 

 modification through the increased use of the middle digit 

 and the relative disuse of the lateral digits. Variations in 

 these digits are no longer suppressed and eliminated. Any 

 congenital predisposition to increased development of the 

 mid-digit, and decreased size in the lateral digits, will tend 

 to assist the adaptive modification and to supplement its 

 deficiencies. Any congenital predisposition in the contrary 

 direction will tend to thwart the adaptive modification 

 and render it less efficient. The former will let adaptive 



