114 EVOLUTION OF TO-DA Y. 



justify the statement that the fossil history since the 

 Silurian age is in strict accordance with what we 

 should expect to find according to the theory of 

 evolution. Scientists to-day who have carefully 

 studied the subject, claim that paleontology practi- 

 cally demonstrates the derivative origin of species. 

 It shows as far as it is possible to be shown that our 

 fauna is derived from that of the geological ages ; 

 our classes being descendants of the older general- 

 ized forms, our families from later types, and our 

 species from the very recent branches of the later 

 divisions. 



The Development of tlie Brain in Mammals. 



One very interesting result has appeared from the 

 work of Marsh upon tertiary mammals in regard to 

 the growth of the brain. To give his own summary : 

 (i) All tertiary mammals had small brains. (2) 

 There was a gradual increase in the size of the 

 brain during this period. (3) The increase was 

 mainly confined to the cerebral hemispheres or 

 higher portions of the brain. (4) In some groups 

 the convolutions of the brain have gradually become 

 complicated. (5) In some the cerebellum and 

 olfactory lobes have diminished in size. These are 

 very significant results when we consider the enor- 

 mous size of many of the older animals and the 

 comparative small size of the modern ones. It ap- 

 pears that till the time of the appearance of mam- 

 mals the struggle for existence had been confined 

 to physical superiority ; but that, with the beginning 

 of the Tertiary, a new era ensued, resulting in the 



