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 
