Man in the Light of Evolution 



must trace their quite distinct origin, and then 

 we can appreciate their great power when 

 united. 



The amount which any animal can devote to 

 reproduction is the balance left after the needs 

 of the body for repair and fuel have been met. 

 Our muscular and nervous engines consume a 

 large amount of the food which we can digest. 

 The balance is very large In the stationary or 

 sessile animal, smaller in the sluggish, greatly 

 diminished in the active. It is greater in small 

 animals than in large ones. Vertebrates are 

 large and active, and mammals and birds are the 

 most active of all vertebrates. The evolution 

 of the muscular system, while increasing the sup- 

 ply of food, has greatly decreased the balance 

 left for reproduction. 



While the amount which can be devoted to 

 reproduction has steadily decreased, the amount 

 required to produce a new individual has In- 

 creased just as steadily. Every egg must contain 

 enough nutriment to build and develop an in- 

 dividual capable of taking and digesting food 

 and of further growth. To build a sponge or 

 a hydra requires very little. A worm Is more 

 expensive. A fish costs even more. The frog's 



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