94 ESSAYS OF A BIOLOGIST 



phant, the baboon. In fact, once the bodily special- 

 ization of units has reached a certain pitch, progress, 

 as we have seen, is only possible through mental de- 

 velopment, and this in the great majority of cases 

 brings about aggregation into some sort of commu- 

 nity, held together by mental bonds. 



Besides aggregation of similar units, there has fre- 

 quently been co-operation between units of unlike 

 character and origin — witness symbiosis, as in li- 

 chens; the relation between many insects and flowers; 

 the formation of flocks consisting of two or more 

 species, as with jackdaws and rooks, and many other 

 cases. 



Competition and co-operation both occur through- 

 out the whole of evolution: but co-operation comes 

 to play an ever more considerable part in higher 

 forms. In lower organisms enormous overproduc- 

 tion is of no great consequence; their organization is 

 simple, and, given favourable conditions, they can 

 turn inorganic matter into their own specific sub- 

 stance at a great rate. But higher forms are more 

 complex, more delicately balanced, and longer lived. 

 Accordingly, waste of life is of greater consequence 

 to them, and methods by which a struggle on the 

 grand scale can be minimized tend to be more and 

 more adopted. We find regularly, for instance, a re- 

 duction of the number of offspring in higher groups 

 together with greater parental care. 



Thus co-operation, for still fresh reasons, is bio- 

 logically important for the higher groups. The prob- 



