170 EVOLUTION 



Karl Pearson draws a distinction between 

 " secular selection," which is Darwin's natural 

 selection, and " periodic selection," which is 

 less easily detected. The difference is this : 

 in the ordinary process of natural selection 

 a change in the mean value of the selected 

 character must be effected from one generation 

 to another. But it might also happen that 

 the extreme deviations from the mean e.g. the 

 giants and the dwarfs were cut off, while 

 the mean value of the character, e. g. the 

 average stature, remained unchanged. This 

 is "periodic selection," whose reality Weldon 

 proved in the case of one of the snails : it can 

 be detected by the diminution in the extent 

 or range of variability. 



There are other extensions of the selection- 

 idea. Thus in social insects like ants, where 

 community competes with community, we 

 see an adumbration of the intersocietary 

 struggle and selection which we are familiar 

 with in mankind. There is also some measure 

 of *' struggle of parts within the organism," 

 as Roux called it, i. e. between competing 

 organs and tissues and cells; and where there 

 is rapid multiplication of elements and discri- 

 minate destruction there must be a definite 

 selective process. There is also a well-known 

 struggle between potential ova, clearly illus 



