88 THE SOCIAL LIFE OF ANIMALS 



tozoans. These have been given in some detail, which 

 has not left time for similar demonstrations among 

 regenerating cells of sponges; nor have I time to tell 

 how hydra have been saved from depression periods 

 by the use of self-conditioned water. I have men- 

 tioned but not elaborated the fact that grouped ani- 

 mals frequently have different rates of respiration as 

 compared with their isolated fellows. This has been 

 recorded widely in the animal kingdom, notably 

 among planarians, certain lower crustaceans, some 

 starfish, fishes and lizards, and for some, at least, asso- 

 ciated survival values have been demonstrated. To 

 this extent, then, I have given the crucial evidence 

 I promised earlier that a sort of unconscious co- 

 operation or automatic mutualism extends far down 

 among the simpler plants and animals. 



These charts should illustrate one other point. The 

 insects stand at the apex of one long line of evolu- 

 tion; mammals and birds are at the peak of another 

 line of evolution; the two have been distinct for a 

 very long time. This view of evolution indicates that 

 the ancestral tree of animals is not like that of a pine 

 tree with man at the very top and insects and all the 

 other animals arranged as side shoots from one main 

 stem. Rather, there are at least two main branches 

 which start low, as in a well-pruned peach tree. Both 

 rise to approximately equal heights, indicating cor- 



