The Development of the Animal World 83 



a colossal task which we must wholly avoid here. We 

 turn rather to the second type of reptile that adapted 

 itself to the changed conditions, and study the more 

 interesting evolution of the mammals. 



Australia cannot be said to be a conservative country 

 since Dutch and Englishmen peopled it, but from the 

 zoological point of view it is a conservative paradise. 

 There we find the most primitive fishes, the most 

 primitive lizards, and the most primitive of mammals and 

 of natives. The duck-mole of Australia is what Darwin 

 called "a living fossil." It connects the mammal and 

 the reptile in a remarkable way. The punctures in its 

 breast, through which the fat oozes, to be licked off by 

 the young, just entitle it to the name of mammal 

 ("mamma" means breast), but it lays eggs like the 

 reptile, has a common outlet for its excreta, and other 

 reptilian features. It is not only — with a few similar 

 forms — the lowest type of existing mammal, but it 

 illustrates remarkably the evolution of the mammal from 

 the reptile. The earliest mammal skeletons we have are 

 described as those of small insect-eating Monotremes 

 ("with one outlet," like the duck-mole and spiny ant- 

 eater). Recent geologists are inclined to think the 

 evolution took place on the now sunken continent 

 between Brazil and Africa, and there are, as a matter of 

 fact, reptile skeletons found in South Africa which many 

 identify as the ancestors of the mammaL 



It is in the Jurassic strata that we first find these fate- 

 ful little creatures that were destined to replace the 

 colossal reptiles as lords of the earth. Their coat of 

 hair fitted them to oppose the cold, their meagre insect 

 diet enabled them to be independent of the decay of the 

 luxuriant vegetation, and their four-chambered hearts 

 supplied a richer and warmer blood to their frames. 

 Partly from this richer supply of blood in a small frame, 



