26o DIVERSIONS OF A NATURALIST 



distinguish it both from the tertiary period, when 

 mammals were abundant and large, and from the 

 Palaeozoic or primary period, at the end of which terres- 

 trial vertebrates first began to make their appearance. 

 These huge reptiles — such as the Iguanodon, the 

 Triceratops, and the Diplodocus (all to be seen in 

 skeleton, though not in the flesh, at the Natural History 

 Museum) — had brains of an incredibly small size, much 

 smaller in proportion to their bulk than those of living 

 reptiles, such as lizards and crocodiles. The same extra- 

 ordinary difference of size of brain is seen when we 

 compare the large living mammals with their equally 

 large extinct forerunners in the early tertiary strata. 

 The skulls and whole skeletons of great rhinoceros-like 

 animals — some of them ancestrally related to our living 

 rhinoceroses — are dug up in early tertiary sands and 

 clays, which have absurdly small brains. We can take a 

 mould of the interior of the brain cases of these extinct 

 animals and compare them with that of the recent 

 rhinoceros. We find that the extinct animal's brain was 

 in many cases only one-eighth the bulk of that of its 

 modern representative 1 



The same disproportion in the size of the more 

 ancient animal's brain is found when we compare the 

 brain of the modern horse with that of its early tertiary 

 ancestors. The modern animal has, as a rule, a very 

 greatly increased size of brain when compared with its 

 Miocene forefather. In fact, it seems that the brain has 

 had, as it were, an independent development in several 

 lines of descent, and whilst the rest of the structure of 

 the ancestral form has been only slightly modified in its 

 proportions, the brain cavity and the brain within it has 

 enormously increased. It is therefore not so exceptional 

 a thing as it at first appears — but only an instance of a 



