254 THE LESSON OF EVOLUTION 



hand. Again, man could not have attained civili- 

 sation if he had not been able to domesticate animals 

 .and to cultivate food-plants. Buminant mammalia 

 were therefore required ; and these can only exist in 

 large flocks, through the peculiar growth of the 

 leaves of grasses on which they feed. Most leaves 

 grow very rapidly after the bursting of the bud, and 

 then cease to grow altogether. The consequence is 

 that if the leaves of one of these plants are con- 

 tinuously cut or pulled off, they are not reproduced, 

 and the plant dies. But in the grasses and their 

 -allies the leaves continue to grow at their bases all 

 through life, so long as the temperature and mois- 

 ture of the soil are favourable ; and cutting and biting 

 off their ends does the plant good instead of harm, 

 for it exposes the newly grown parts of the leaves to 

 the sun. Thus large herds of animals are enabled to 

 live together without destroying the vegetation ; and 

 it was this that tempted primeval man to leave the 

 forest and live on the open land. 



Now hoofed mammals required a long time for 

 their development ; and, if they had not been a very 

 arly branch of the eutherian stock, they would not 

 have been ready for man to domesticate at the close 

 of the pleistocene period. We have thus no less than 

 five different groups of plants and animals, which 

 must precede man in a certain order, to allow the 

 possibility of human civilisation. Phanerogamous 

 trees and birds must precede the earliest Primates. 

 Grasses and ruminants must follow ; yet they also 

 must precede man. Now we find that this is just 

 the order in which they did appear. Phanero- 



