202 THE MEANING OF EVOLUTION 
next higher class of backboned animals. Here very 
distinct developments of the process are discovered. 
The turtle, to use the best known illustration, may 
lay but twenty eggs. But she will not lay them at 
random in the water, as do the toads and the fish. 
Each egg is wonderfully fattened with yolk. This 
means that it is possible for the creature to develop 
to a far greater extent before leaving the egg than 
was possible in the case of the toad. Accordingly 
the little turtle, while it begins life not unlike a fish 
and goes through the gilled and tailed period, during 
which it is not unlike a tadpole, passes beyond this 
period before leaving the shell and has already ac- 
quired its full turtle characters when first it steps upon 
the scene. So big an egg as this would be highly 
nutritious and animals would desire it immensely for 
food. Hence it becomes necessary for the turtle to 
securely hide her eggs. In order to do this, she 
scoops out a pit in the sand in which she deposits 
them and here they develop. If no further provi- 
sions were made the eggs of the toad would dry com- 
pletely and never hatch. Accordingly it becomes 
necessary for the turtle to enclose each egg in a tough, 
leathery membrane, known as the shell. Because the 
ege is thus encased it is necessary for it to be fertil- 
ized before being laid. Accordingly the male must 
place the sperm cells within the body of the female. 
