Page:Popular Science Monthly Volume 22.djvu/767

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tropics, and even in colder countries glide rapidly along in the warm sunshine, or hide in nooks and crannies, and sleep the winter away; and the birds the merry, active, warm-hearted birds—live everywhere.

Yet still the great backboned division is not exhausted; on the contrary, the most powerful if not the most numerous group is still to

PSM V22 D767 First known mammals.jpg
Fig. 1.

come—the mammalia, or milk-giving animals. Let us first notice two important changes which give them an advantage over other back-boned creatures. We have found the fish casting their eggs out into the water, and, as a rule, taking no more thought of them; so it was again with the frogs, so with the reptiles, whose eggs, even when carefully buried by the mother, are often devoured by thousands before the little ones have a chance of creeping out of the shell. And with the birds, in spite of the parents' care, more eggs probably are eaten by snakes, weasels, field-rats, and other creatures, than remain to be hatched.

Now, the cat and the cow, as we all know, do not lay eggs as birds do; but the mother carries the young within her body till they are born, perfectly formed, into the world. And when at last her little