GENERAL LIFE OF MAMMALS. 677 



it is the uterine mucosa which forms the maternal part of the placenta. 

 Into the vascular syncytium the allantoic capillaries grow down, until 

 ultimately maternal and fcetal vessels are separated merely by their 

 endothelial walls and a mere trace of syncytial protoplasm. The 

 connection between the yolk-sac wall and the uterus is effected in a 

 similar manner. 



General Life of Mammals. 



Most Mammals live on dry land. The bats, however, 

 have the power of flight, and various forms are able to 

 take long swooping leaps from tree to tree. Thus there are 

 " flying phalangers," such as Petaurus, among Marsupials ; 

 " flying squirrels," such as Pteromys, among Rodents ; 

 " flying lemurs " (Galeopithecus), allied to Insectivores. Not 

 a few are aquatic, — all the Cetaceans, the two Sirenians, and 

 the Pinniped Carnivores, such as seals and walruses ; while 

 water-voles, beavers, otters, polar bear, and many others 

 are also at home in the water. Burrowers are well repre- 

 sented by moles and rabbits ; arboreal forms by squirrels 

 and monkeys. 



As to diet, man, many monkeys, the pigs, and many others, 

 may be called omnivorous ; kangaroos, hoofed animals, and 

 most rodents are herbivorous ; the Echidna, the ant-eaters, 

 hedgehogs and shrews, and most bats, are insectivorous ; 

 most of the Carnivora are carnivorous ; dolphins and seals 

 feed chiefly on fishes ; but in most cases the diet varies not 

 a little with the available food-supply. 



The struggle for existence among Mammals is sometimes 

 keen among fellows of the same kind ; thus the brown rat 

 {Mus decumanus) tends to drive away the black rat (M. 

 rattus) ; but stress, due to over-population, is sometimes 

 mitigated by migration, as in the case of the lemmings. The 

 struggle seems to be keener between foes of different kinds, 

 between carnivores and herbivores, between birds of prey 

 and small mammals ; but combination for mutual defence 

 often mitigates the intensity of the conflict. Teeth and 

 claws, hoofs and horns, are the chief weapons, while the 

 scales of pangolins, the bony shields of armadillos, the spines 

 of hedgehogs and porcupines, and the thick hide of the 

 rhinoceros, may be regarded as protective armature. In 

 keeping their foothold some Mammals are helped by the 



