GENERAL CHARACTERISTICS 13 



place among the inhabitants of the earth. It was, indeed, during this so-called 

 Tertiary period that these animals attained the dominant position which they now 

 occupy ; and the present stage of the earth's history may be truly called the age of 

 Mammals and Birds. We are not, however, to suppose from this that Mammals 

 were unknown before the Tertiary period ; a considerable number of species, mostly 

 of small size, having been already discovered. 



An additional importance attaches to the study of extinct Mammals, since it is 

 by their means alone that we are able to explain several apparent anomalies in the 

 geographical distribution of living groups. How, for instance, could we possibly 

 explain the present existence of tapirs only in such widely remote areas as the 

 Malay Peninsula and Islands and South America, unless we had learned by geological 

 explorations that these animals formerly roamed over large portions of Europe and 

 Asia, from whence their descendants gradually migrated to the regions where they 

 now remain? 



The former occurrence of an epoch of great cold in the northern hemisphere 

 known as the Glacial period, furnishes us with an explanation of how nearly related 

 animals are now confined to isolated mountain chains ; their ancestors having been 

 enabled, during the prevalence of the cold, to spread over the plains of the temperate 

 regions, from whence they retreated with the advent of warmer conditions to seek a 

 congenial climate in the nearest mountain region. 



Mammals may be divided into eleven main groups or orders, which 



may be arranged as follows, and will be treated of in the same se- 

 Mammals 



quence, viz.: 



1. Apes, Monkeys, and L,emurs PRIMATES. 



2. Bats CHIROPTERA. 



3. Insectivores INSECTIVORA. 



4. Carnivores CARNIVORA. 



5. Hoofed Mammals UNGULATA. 



6. Manatis and Dugongs SIRENIA. 



7. Whales and Porpoises CETACEA. 



8. Rodents RODENTIA. 



9. Sloths, Anteaters, etc. EDENTATA. 



10. Pouched Mammals MARSUPIALIA. 



1 1 . Egg-laying Mammals MONOTREMATA. 



It is not to be supposed that all these groups are separated from one another by 

 differences of equal importance. For instance, No. 10 differs from the preceding 

 groups by characters of far more importance than do any of those nine from one an- 

 other ; while the members of No. 1 1 differ fundamentally, not only from the first 

 nine groups, but almost equally markedly from No. 10. 



Having said thus much by way of introduction, we proceed to the consideration 

 of the first order of Mammals. 



