THE GENEALOGY OF ANIMALS 89 



of the wind, as do the birds ; nor bury themselves 

 in subaqueous mud, as do the frogs, snakes, 

 and crustaceans. During the Mesozoic period, 

 when cold - blooded reptiles of gigantic size 

 flourished over a wide area of the earth's surface, 

 the planet was far warmer than now. Animals, 

 therefore, like the mammals (or birds), capable of 

 maintaining a fixed temperature regardless of the 

 thermal fluctuations of the surrounding media, 

 are the only animals of large size and power 

 capable of uninterrupted existence over the 

 greater part of the surface of the existing earth. 

 The pre-eminent life of the Cenozoic time was 

 mammalian. But the decline and fall of the 

 saurian power was not wholly due to the rise of 

 the more dynamic mammals. It was in part due, 

 no doubt, to adverse conditions of climate, and 

 also to the fact that mammals and birds guard 

 their eggs, and saurians do not. 



The lowest of the mammals are the monotremes, 

 animals which blend in a marvellous manner the 

 characteristics of birds, reptiles, and mammals. 

 Only two families of these old-fashioned creatures 

 are left, the echidna and the duck-bill (ornitho- 

 rhynchus), both of them found on or near that 

 museum of biological antiquities, Australia. They 

 are covered with hair and suckle their young like 

 other mammals, but they have only the rudiments 

 of milk glands, and they lay eggs with large yolks 

 from a cloaca, like the reptiles and birds. The 

 duck-bill hides its eggs in the ground, but the 

 echidna hatches its eggs in a small external 



