458 The Outline of Science 



every open door of opportunity. This is what Professor H. F. 

 Osborn calls "adaptive radiation," and he distinguishes as many 

 as twelve habitats. ( 1 ) There are terrestrial mammals, walking 

 like the elephants, running like the antelopes, jumping like the 

 kangaroos. (2) But some are burro wers as well as runners, as 

 rabbits well illustrate. (3) Then there are thoroughgoing bur- 

 rowers, like the moles, which have conquered the underground 

 world. (4) Some are as much at home in water as on dry land; 

 we think of the roving otter and the polar bear. (5) Perhaps 

 a separate division may be made for those mammals that frequent 

 streams, after the manner of beavers and the familiar water-vole 

 which can hardly be saved from its popular name of "water- 

 rat." (6) The shore of the sea is the habitat of seal, sea-otter, 

 and walrus. (7) The open-sea mammals are the cetaceans large 

 and small, from whale-bone whale to porpoise. (8) Professor 

 Osborn takes the deep-diving finback whales as examples of 

 mammals that actually explore the great abysses, but this is per- 

 haps stretching a point. (9) Then there are the betwixt-and- 

 between mammals transitional between arboreal and terrestrial 

 life, like the macaque monkeys and the gorilla. (10) Strictly 

 arboreal types are well represented by squirrels, tree-sloths, and 

 lemurs. (11) The volplaning "flying squirrels" and "flying 

 phalangers" form another interesting betwixt-and-between 

 group, essaying the conquest of the air in their daring parachut- 

 ing from tree to tree. (12) Finally, the bats are true fliers- 

 aerial mammals. 



It is useful to recognise this variety of habitat, for it shows 

 how diverse the life of mammals must be, and the impression of 

 diversity grows when we remember that in most habitats there 

 are several distinct possibilities of food-getting. Thus a mole 

 is a carnivorous burrower, while a vole is a vegetarian bur- 

 rower; a small bat is an insectivorous flying mammal, while 

 a big bat is usually a fruit-eater. It is very interesting to 

 find that almost every haunt and diet illustrated by mammals 



