498 The Outline of Science 



fish; unless we understand the general features of mammals we 

 will not appreciate his insight. 



The dominantly successful orders of present-day mammals 

 are: (1) the Carnivores (cats, dogs, bears, seals, etc.); (2) the 

 hoofed Ungulates (horses, tapirs, rhinoceros, cattle, pigs, hippo- 

 potamus, camels, perhaps also including the elephants) ; and (3) 

 the monkeys and apes or Primates. These represent three great 

 lines of evolution. On the Carnivore line the premium is on teeth 

 and claws, quick senses and alert movements. On the Ungulate 

 line the premium is on swiftness, on the power of covering long 

 distances in search of herbage, and on such weapons as horns and 

 hoofs, rendered more effective still when their possessors are gre- 

 garious. On the Primate line the premium is on the power of 

 climbing, the emancipated hand, and the restless brain. Below 

 the level of true Primates are the Lemurs, or half -monkeys 

 ghost-like nocturnal creatures, mostly confined to the forests of 

 Africa and Madagascar. 



Not very far off the Carnivore line of evolution, but much 

 more primitive, is that of the Insectivores (e.g. moles, hedgehogs, 

 and shrews) ; and the Bats with their power of flight must be 

 regarded as the specialised descendants of arboreal Insectivora. 



Balancing the Insectivores there is the somewhat humble 

 order of Rodents, on a quite different evolutionary tack, the rats 

 and mice, squirrels and porcupines, rabbits and hares. The 

 toothed Whales and baleen Whales are mammals that have taken 

 secondarily to marine life, and are as specialised for swimming 

 and diving as bats are for flight. And besides these well-known 

 orders, there are the sea-cows or Sirenians (including nowadays 

 two genera only, the dugong and the manatee) , the old-fashioned 

 Edentates (sloths and armadillos, ant-eaters and pangolins), 

 perhaps to some extent survivors of the archaic mammals. But 

 more primitive in their affinities than all these are the Marsupials 

 (mostly confined to Australia) ; and lowest of all are the egg- 

 laying Monotremes, also Australasian. 



