THE TRIUMPH OF THE MAMMALS 93 



an adaptation to a colder climate. They mean warm 

 blood and care of the young. 



We saw how the first mammals arose. One pro- 

 gressive branch of the reptiles, which passed safely 

 through the sieve of natural selection during the 

 Permian Ice Age, was capable of being adapted to the 

 colder climate. It is little use to give you its name 

 here, though scientific men are now fairly decided 

 which branch of the ancient reptiles it was that took 

 this fortunate turn. The heart became four- 

 chambered, and the blood warm. But again there 

 was no sudden leap; in fact, there is not in nature 

 to-day a perfectly clean distinction between the lowest 

 mammals and the higher reptiles. "Warm blood" 

 means, as I said before, that the blood system has the 

 power to keep the blood at a fairly even temperature 

 while the temperature outside may vary considerably. 

 In plain English, it means having warm blood in cold 

 weather. The blood of the reptile sinks with the 

 temperature of its surroundings. That is the chief 

 reason of the winter sleep of the tortoise or the snake. 

 But the lower mammals have not entirely "warm 

 blood." Their blood varies as much as 30° F. in 

 temperature. They live in Australia and New 

 Guinea, and are therefore not inconvenienced by 

 a severe winter. This imperfectness of their 

 machinery is quite a good illustration of evolution. 



The fur coat, or coat of hair, is fully developed in 

 every mammal that we know, and we can only guess 



