116 ANIMAL INDIVIDUALITY [CH. v 



pressure with pressure, and kept the first species in 

 its place by themselves too discovering means of 

 adding to their independence. While the balance 

 of power lasts, variation no doubt takes place, but 

 there is no strong necessity to guide it. Once let 

 a large, favourable variation take place in a species, 

 however, so giving it a handicap, and then for its 

 competitors natural selection is at once made more 

 active they must perish or else adjust themselves 

 by a variation, generally in a similar direction. So 

 it comes to pass that the continuous change which 

 is passing through the organic world appears as 

 a succession of phases of equilibrium, each one on 

 a higher average plane of independence than the one 

 before, and each inevitably calling up and giving 

 place to one still higher. 



This digression was necessary to give some ex- 

 planation of the succession of ever more perfect 

 individualities, and the continual repetition of the 

 same methods in their attainment. 



Space forbids more than the merest outline of the 

 developments of individuality after it has attained 

 its second grade. We have seen (p. 64) that the 

 method of aggregate differentiation is now for a time 

 the less important : it still, however, exhibits some 

 interesting points. 



To start with, division of labour in colonies of 

 second-grade individuals, as in colonies of cells, 



