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 

 egg 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. 



