THE LESSON OF EVOLUTION 29 



the lower Jurassic. Birds in the upper Jurassic. The 

 Primates and primitive hoofed mammals in the lower 

 eocene. Grasses in the oligocene, Euminants in the 

 miocene, and man in the pliocene. The mesozoic 

 mammals were all quite small, and we do not know 

 the structure of their feet, so we cannot say whether 

 they were arboreal or not ; but, with this possible ex- 

 ception, \ve find that the different classes came into 

 existence just when they were wanted. 



We must remember that these groups of plants and 

 animals form widely separated branches of the tree of 

 life, and that the necessary correlations, of which I 

 have been speaking, lie outside the jurisdiction of 

 natural selection ; which, although it regulates the 

 development of each branch, has no power of co- 

 ordination between two branches, unless one forms the 

 food of the other. So that there is no reason at all 

 why they should have been developed in the particular 

 order in which they appear. 



For example, the origin of birds depends chiefly on 

 the development of highly complex papillae in the 

 skin, from which the feathers are formed. If these 

 had not been developed in the naked skin of a reptile, 

 flying birds would never have come into existence. 

 And if there had been no birds, or even if their origin 

 had been delayed until the miocene period, there would 

 have been no monkeys nor man. So also, if no 

 Euminants had been developed, this would not have 

 prevented the appearance of apes or even of man ; but 

 man would have remained in the stage of a hunter 

 all his days, and could not have lived in large 

 communities. 



Now, if there had been only two of these groups, we 



