190 THE WORLD OF LIFE chap. 



more accurately determined, but the abrupt change in the 

 life-forms, and the world-wide unconformity in the stratifica- 

 tion on passing from one division to the other, are as great as 

 ever. The Primary or Palaeozoic period is still that of fishes 

 and Amphibia ; the Secondary or Mesozoic, that of reptiles, 

 in amazing abundance and variety ; and the Tertiary or 

 Cainozoic, that of an almost equal abundance of Mammalia, 

 and with a considerable variety of insects and birds. 



The exceptions to the generality of this classification are 

 few, and are particularly interesting. Of the myriads of 

 reptiles that characterise the Secondary era, only two of the 

 nine orders into which they are subdivided have been found 

 so far back as the Permian, the latest of the Palaeozoic 

 formations. One of these most primitive reptiles has a near 

 ally in the strange, lizard-like Hatteria still surviving in some 

 small islands on the coast of New Zealand ; while others 

 which seem to form connecting links with the earliest 

 mammals may be the ancestral form from which have 

 descended the unique types of the lowest living Mammalia, 

 the ornithorhynchus and echidna of Australia. 



So with the highest type of vertebrates, the mammals. 

 About the middle of the nineteenth century small mammalian 

 jaws with teeth were discovered in what was known as the 

 dirt-bed of the Purbeck (Jurassic) formation at Swanage ; 

 others in the Stonesfield Slate of the same formation ; and at 

 a later period very similar remains were found in beds of the 

 same age (and also in the Cretaceous) in North America. 

 These are supposed to be primitive insect-eating Marsupials 

 or Insectivora, and were all about the size of a mole or a rat ; 

 and it is a striking example of the imperfection of the 

 geological record, that although they occur through the 

 whole range of the Secondary period, from the Trias to the 

 Cretaceous, their remains are still exceedingly scanty, and 

 they appear to have made hardly any structural progress in 

 that enormous lapse of time. Yet directly we pass from 

 the Cretaceous to the Tertiary rocks, not only are Mammalia 

 abundant and of fairly large size, but ancestral types of all 

 the chief orders occur, and such highly specialised forms as 

 bats, lemurs, and sea-cows (Sirenia) are found in its earliest 

 division, the Eocene. 



