Altogether a higher state of reproduction is encountered when we reach the reptiles, which are the 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 acquired 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 provisions were made the eggs of the turtle would dry completely 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 fertilized before being laid. Accordingly the male must place the sperm cells within the body of the female. These cells swim nearly to the top of the tubes in which they are placed, and there fertilize the descending eggs. Farther down the canal the shell is secreted about the now swollen mass of yolk and white, completing the egg just before it leaves the parent. If the evolutionist understands properly the line of descent, the birds and mammals are both the descendants of the reptiles. While there is less exterior resemblance between a chicken and a turtle than between a cat and a turtle, the real relationship in the first case is much closer than in the second. This is perhaps most easily seen in the scaly legs of both bird and reptile. Another remarkable resemblance lies in the fact that in both cases the eggs are large, well stored with nourishment, and protected by a resistant shell.