92 ESSAYS OF A BIOLOGIST 



have been based on the co-operation of units — the 

 origin of multicellular from unicellular organisms, 

 and the development of true man, with his social life, 

 from his pre-human ancestor. It is also prominent 

 in the lives of many species of the highest groups — 

 insects, mammals, and birds : witness the ants and 

 bees, the rook, the wild dog, the elephant, the baboon. 

 In fact, once the bodily specialization of units has 

 reached a certain pitch, progress, as we have seen, 

 is only possible through mental development, and this 

 in the great majority of cases brings about aggregation 

 into some sort of community, held together by mental 

 bonds. 



Besides aggregation of similar units, there has 

 frequently been co-operation between units of unlike 

 character and origin — witness symbiosis, as in lichens ; 

 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 over-production is of 

 no great consequence ; their organization is simple, 

 and, given favourable conditions, they can turn in- 

 organic matter into their own specific substance at 

 a great rate. But higher forms are more complex, 

 more delicately balanced, and longer lived. Accord- 

 ingly, waste of life is of greater consequence to them, 



