IN THE GRIP OF AN ICE AGE 57 



increased, and the struggle for life increased, the 

 hardier and more active would tend always to push 

 into the more temperate parts. Any useful variation, 

 such as the overlapping of the scales or an improve- 

 ment of the blood circulation, would be fostered by 

 natural selection. Just as lungs were the great thing 

 needed at the earlier stage, so warmth — for the parent 

 and eggs — is now the chief need. The machinery of 

 natural selection worked, age after age, in develop- 

 ing heating-apparatus. The bird was one result; the 

 mammal was another. 



If we could accept the theory called " Mendelism," 

 which I have mentioned in the first chapter, this 

 would in one sense be easier to understand. Ac- 

 cording to this theory, evolution does not work by 

 very slowly and gradually adding together little 

 changes in successive generations, but by occasionally 

 producing young that differ very materially from the 

 mother. Such things are known, and used to be 

 called "freaks." Many scientists believe that they 

 are sometimes the beginning of new species. If we 

 could imagine a mother with a three-chambered heart 

 (a cold-blooded mother) laying an egg which hatched 

 into a reptile with a four-chambered heart (a warm- 

 blooded animal), or a long step towards it, evolution 

 would be easier. But it is a large order, and the 

 supposition does not seem to be justified by the facts. 

 Most men of science still believe in gradual advances; 

 though they lay far more stress than Darwin did on 



