196 THE MEANING OF EVOLUTION 



of her lowest plants and animals she uses the asexual 

 method of reproduction. As we go higher in the or- 

 ganic world the two-parent method becomes increas- 

 ingly common. When we reach the higher animals, 

 and most of the higher plants, this plan of double 

 parenthood, the sexual method, alone is used. 



In order that we may the more clearly understand 

 how the mammals produce their young and nourish 

 them, we shall begin at the lowest class of the back- 

 boned animals and note how the process is there ac- 

 complished. As we pass upward through the king- 

 dom the method acquires greater complexity. When 

 we finally reach the mammals, what at first seemed an 

 absolutely new process will prove to be, as is all of 

 nature's work with which we are thoroughly ac- 

 quainted, but a modification and an elaboration of 

 some previously existing process. 



Some time ago I was passing the early months of 

 summer by the side of a lake in northern Pennsyl- 

 vania. Near my tent, on the edge of the water, was 

 a wharf from which it was possible to look down into 

 the shallows about the edge of the lake. In early 

 July the bottom began to take on a strange appear- 

 ance. Spots as big as a dinner plate became evident 

 because they were cleaned of the finer sand or mud 

 which is common on the bottom. A close examina- 

 tion showed that each of these circular spots was 



