MAMMALS. 577 



First, before describing any of these anatomical peculiarities, let us 

 consider the distribution of these forms. In America occur the opossums, 

 but, with the exceptions to be noted immediately, nowhere else in the 

 world can you find any more such forms. In Australia, on the other 

 hand, these forms abound, while a few stretch south into the adjacent 

 island of Tasmania, and north into New Guinea and Celebes and a few 

 neighboring islands. Australia, however, is the great centre, and before 

 the arrival of the whites it contained no other terrestrial mammals, with 

 the solitary exception of the dingo, or native clog, which may have been 

 introduced by man. 



Another equally interesting feature is the way in which many of these 

 forms simulate other groups of mammals. Thus we have forms like bears, 

 squirrels, dogs, rats, gophers, and the like ; and if we take the fossil forms 

 into consideration, the lions, too, have their representatives among these 

 curious animals. This resemblance extends not merely to the external form, 

 but affects as well the character of the teeth and of the digestive tract. 

 The reason for this is not far to seek. In the region occupied by those 

 animals all activities of life were open without competition, and one form 

 adopted one habit, and another another, and with it came a change of 

 structure better adapted for the mode of life. Formerly habits were 

 regarded as the results of structure, and many pages have been written to 

 show how every feature of anatomy was adapted to the mode of life. The 

 inference was that forms were created thus, and that habits were the 

 result of structure. Now we know better, and the converse of the old 

 idea is now admitted by all to be true, — structure is secondary, and is 

 produced and modified by habits. 



First in order come the American opossums, typified by the common 

 opossum of the warmer parts of the United States, which is well shown hi 

 the accompanying cut. It is much like a rat in its appearance, except in 

 size. It has the same jDointed snout, the same pointed ears, and the same 

 round scaly tail. This tail, however, has a function unknown to that of 

 the rat ; for it is prehensile like that of some of the monkeys. It can be 

 curled around the limb of a tree with sufficient strength to support the 

 body. The opossum is largely a nocturnal animal. In the day-time it lives 

 in some hollow tree ; but at night it descends, to begin its wanderings and 

 its search after food, largely of insects, varied with an occasional reptile or 

 bird, and, when convenient, with eggs. The Texan variety is said to be 

 very fond of the black persimmon. 



The common opossum has a well-developed pouch in which it carries 

 its young. For the first two weeks the mother keeps the opening of the 

 pouch closed ; but after that time it is opened, and when the young (from 



