62 ANIMAL INDIVIDUALITY [CH. 



among the cells, they are rendered mutually depen- 

 dent, and the colony is transformed into a true 

 individual, which is obviously of a higher order than 

 the cell. It has attained what may be termed the 

 second grade of individuality. 



This method, for want of a better term, I shall 

 call aggregate differentiation, to show that the 

 individual formed by its means consists of an aggre- 

 gation of smaller individuals. It differs from the 

 second method in that division of labour, instead 

 of taking place among the parts of a single unit, 

 affects whole units or even groups of whole units. 



This third method is of special importance for 

 the evolution of life because those organisms that 

 have adopted it have found the only satisfactory 

 solution of that besetting problem how to become 

 large. It is of importance for the understanding of 

 individuality because it gives the clue to many of 

 the apparent paradoxes of the higher organisms or 

 Metazoa why they are built up of units comparable 

 with free-living Protozoa, why they so often reproduce 

 by means of a single cell, why the embryo produced 

 from this single cell so often consists of a number of 

 almost identical cells among which division of labour 

 only later sets in. 



Now that the animal has separate units to build 

 with, each with a firm membrane and definite shape 

 of its own, progress is much more rapid. In the first 



