that do not. Animals and people learn and go up higher in life, in proportion as they are sociable. A mother and her babies are sociable with one another. They love each other and they teach each other. The mama cat learns to be shrewd and careful because she has other little mouths to feed and lives to protect, beside her own. It is just as true in the insect world. For example, the ants and spiders are both very bright. They know how to do many wonderful things, and these wonderful things are most of them done in taking care of their eggs, and the babies that are hatched out of these eggs.
And do you know about how these ants keep other insects for cows? These "cow" insects are the little green lice that you find on plants. They are not good for the plants but they make good "cows." They give down a kind of honey dew, just as the old cow gives milk for her babies. Don't you wonder if this honey dew is meant for the babies of the aphis or plant lice? If it is true that they do give this sweet milk for their babies, they are really mammals, too, and when the higher animals feed their young in this way they are simply repeating something that is done away down in the insect world.
Another odd thing about these aphides is that sometimes they lay eggs, and sometimes they bring forth their babies alive, already hatched. It is when they have wings that they lay eggs but have no milk, and in the state that they bring forth their young alive, they have this milk. So the more we think about it the more it seems as if these little bugs are mammals, too.
But whether the aphis is one kind of a "bird" that suckles its young and so seems to want to remind us still more of the relation between birds and mammals, it is certain that there are egg-laying animals that suckle their young. One of these is the spiney ant-eater. Another is the duck mole. You can see from his name that he must be something like a duck and something like a mole. He burrows in the ground and suckles his young like a mole and he has a bill and lays eggs like a duck.
Then there are fish that suckle their young. They might be called fish because they live in the water, and swim like fish; or they might be called sea-lions because they have sharp teeth and eat meat like dogs or lions, and suckle their young as the lioness does her cubs.
As we find some mammals laying eggs—most of them seem to have dropped their egg-laying habits with their wings—so we find
