Not only in the forms of things, but in the stuff they are made of, Nature does skip. She puts lime in the shells of the crawfish and the oyster, but leaves it out of the shells of insects, which come between them. Not all insects have shells, as you know, but such of them as have shells for their backs, or shell-like scales for wings, do not have any lime in them.
Why do the crawfish and the oyster have lime in their shells, while the insects haven't? Think of the lives they lead and you will see the answer. If the crawfishes and the lobster didn't wear strong armor, what would happen to them in the fighting lives they lead? And what would happen to poor Mr. Oyster, who can't fight at all, if his shell were not still thicker?
Flies, grasshoppers, butterflies and a lot of other little friends of yours, belong to the great insect family. There are several reasons why they are placed higher up than the crawfish family. For one thing they have three distinct regions of the body. In that respect they are more like human beings than the crawfish family.
Insects also have but six legs. "Do one thing at a time, and do it well," seems to be Nature's motto. So, in comparing the inner and the outer forms of different members of the animal world we see special parts developing all the time to do new things—as in the case of wings in birds; or to do old things better, as we see when we come to comparing stomachs.
The earthworm has a very simple stomach. His inside is almost all stomach; just a simple tube that digests all the way down. In the crawfish we see these "insides" are pretty much all tube. But, instead of having a lot of legs, like the earthworm, the crawfish has fewer legs; and these legs differ from each other, and are used for different purposes. The earthworm's legs are all alike and are all used for one purpose—to help him get along over the ground.
In the insects we see still fewer legs. Insects always have three pair of legs, while the crawfish has four pairs. Still higher up in the scale are the animals with only two pairs. You see how Nature makes fewer and fewer legs as she goes up and up? Finally she makes two of these four legs into wings, and lo, a bird! Or she makes them into arms, and behold, a little boy or girl! So the legs grow fewer in number and more useful. Think how much better it is to have two real good legs and a pair of arms, than to have as many legs as the earthworm or the "thousand legged" worm, and no arms or wings or anything like that—just legs, legs, legs.
