Page:A Short History of the World.djvu/52

HESPERORNIS, A MESOZOIC BIRD, IN ITS NATIVE SEASBIRDS by competition and pursuit into a life of hardship and adaptation to cold. With them also the scale became quill-like, and was developed into a heat-retaining covering; and they too underwent modifications, similar in kind though different in detail, to become warm-blooded and independent of basking. Instead of feathers they developed hairs, and instead of guarding and incubating their eggs they kept them warm and safe by retaining them inside their bodies until they were almost mature. Most of them became altogether viviparous and brought their young into the world alive. And even after their young were born they tended to maintain a protective and nutritive association with them. Most but not all mammals to-day have mammæ and suckle their young. Two mammals still live which lay eggs and which have not proper mammæ, though they nourish their young by a nutritive secretion of the under skin; these are the duck-billed platypus and the echidna. The echidna lays leathery eggs and then puts them into a pouch