154 



THE POPULAR SCIENCE MONTHLY 



Didelphidae (opossums) have a well-developed thumb ; in some of the 

 Dasyuridie it becomes very small, while a tolerably distinct thumb 

 characterizes the Phascogales ; a rudimentary thumb in Dasyurus ; 

 no external thumb in _Z>. Mavjei, but its metatarsal exists, while in 

 Thylacinus even its metatarsal is gone. 



Fig. 4. A, Young Female Oi>ossum {DMelphyn Virginiana). Natural Size. B, Marsupium, 

 clitoris, and vent of the same, enlarged ; C, Marsupium, penis, and vent of a male ol' the 

 same litter, enlarged. 



Below the marsujiials stands the group of Monotremes, including 

 the remarkable Australian Ornithorhynehus and Echidna. In the 

 former the openings of the milk-glands on the abdomen are not marked 

 by any elevation or depression ; but in Echidna we find a similar pair 

 of glands, the opening of each becoming depressed at maturity, so as 

 to form a small pit, into which the nose of the young is inserted and 

 attached, where it remains pendant and nourished while its develop- 

 ment advances. This pair of little pits may be regarded as the be- 

 ginning of the bilateral pocket so largely developed in some marsu- 

 pials. If we can imagine that these depressions have become so deep 

 as to envelop not only the nose of the young, but also its whole body, 

 we can understand the evolution of a marsupial from something lower. 

 At the same time we should notice that these depressions are just the 

 opposite of what we find in the higher mammalia, where the mamma- 

 ry glands form larger or smaller abdominal or pectoral prominences. 

 The milk-glands of Ornithorhynehus seem j^rimitive, while the de- 

 pressed glands of Echidna and the marsupials, and the elevated glands 

 of higher mammals, may be viewed as differentiations of the same. 



The opossum is the animal on which the first observations of mar- 

 supial repi'oduction were made. At first the young, found in an im- 

 perfect condition Avithin the pouch, were not examined closely enough 

 to disclose their real nature. They were regai'ded as formless and 

 inanimate. Even in the " Natural History of New York," Part I,, 

 the young is spoken of as " a small gelatinous body, not weighing- 

 more than a grain." But these ideas of the early observers still exist 

 in the popular mind, and are as imperfect as their explanations as to 

 how the young originated. The peculiar character of the young led 

 to the belief that they must have developed from the parents' teats, 

 by a kind of metamorphosis or budding process. This gemmiparous 



