370 ACTION OF NATURAL SELECTION 



especially likely to eliminate, are still those which were 

 originally, as the result of blastogenic variation, re- 

 spectively larger and smaller than the average. The 

 selected individuals are therefore not only larger in 

 themselves, but in that their " largeness " is to some 

 extent a blastogenic variation, their offspring will also 

 be, on an average, larger than the normal. 



It is not intended to imply that increased variability 

 is by any means always an advantage. In a stable 

 form, upon which Selection is acting but little, it might 

 be a distinct disadvantage, as the more variable indi- 

 viduals might be less adapted to their environment 

 than the less variable. In the case of the sparrow, for 

 instance, we saw that Bumpus found that the extreme 

 individuals in either direction tended to be weeded out, 

 though there was a much greater elimination of the ex- 

 treme individuals in one direction than of those in the 

 other. What is true for one form, however, is by no 

 means necessarily true for another, and in a rapidly 

 evolving organism, such as the pale-coloured mouse 

 found on the sand-banks off Dublin, it is probable that 

 the eliminated individuals would be chiefly confined to 

 the darkest specimens, and include but few of the palest 

 ones. 



