VERTEBRATES: BACKBONE AND BRAIN 85 



organs, and their activity was greatly heightened. As 

 more and more heat was produced by the combustion in 

 muscular and nervous tissues, and less was lost by con- 

 duction, the temperature of the body rose, and in birds 

 and mammals becomes constant several degrees above 

 the highest summer temperature of the surrounding 

 air. 



The changes in the brain affect mainly the large and 

 small brain. The cerebellum increases with the greater 

 locomotive powers of the animal. But its develop- 

 ment is evidently limited. The large brain, or cere- 

 brum, is in fish hardly as heavy as the mid-brain ; in 

 amphibia the reverse is true. In higher recent rep- 

 tiles the cerebrum would somewhat outweigh all the 

 other portions of the brain put together. In mammals 

 it extends upward and backward, has already in lower 

 forms overspread the mid-brain, and is beginning to 

 cover the small brain. But this was not so in the ear- 

 liest mammals. Here the cerebrum was small, more 

 like that of reptiles. But during the tertiary period 

 the large brain began to increase with marvellous 

 rapidity. It was very late in arriving at the period of 

 rapid development, but it kept on after all the other 

 organs of the body had settled down into comparative 

 rest, perhaps retrogression. 



We have given thus a rapid sketch in outline of the 

 changes in the most characteristic systems between fish 

 and mammals. Some of the changes which took place 

 in mammals were along the same lines, but one at 

 least is so new and unexpected that this highest class 

 demands more careful and detailed examination. 



The mammal is a vertebrate. Hence all its or- 

 gans are at their best. But mammals stand, all things 



