OPOSSUMS AND THEIR YOUNG. 153 



intended to show how groups are distinct from each other ; but, if it 

 is equally fair to arrange those characters in such a way as to show 

 the athnities of groups with each other, and what they have in com- 

 mon, we may say briefly that the placental mammals are connected 

 with the marsupials by having 1. Nipples; 2, Free clavicles; 3. An 

 embryonal cloaca, and by these characters both groups are distin- 

 guished from the monotremes below them ; the ijiarsupials and mono- 

 tremes are united by having in common 1. Marsupial bones; 2. Un- 

 developed bigeminal bodies; 3. No placenta, and by these characters 



Fig. 3. Young Opossum. Natural Size. 



are distinguished from the placental mammals above ; while the mono- 

 tremes join with the reptiles in possessing 1. United clavicles ; 2. A 

 permanent cloaca ; 3. No nipples, and by these characters are distin- 

 guished from the marsupials above. A great many more characters 

 and facts from the comparative anatomy, embryology, and palseology, 

 could have been used in this comparison ; but those given are enough 

 to show how characters usually regarded as distinctive only may also 

 at the same time be viewed as connective. 



The order of living marsupials presents remarkable diversity of 

 structure and habits, containing herbivorous, insectivorous, and car- 

 nivorous species ; yet we find all these traits combined in one and 

 the same species, the opossum. It is probable that, by adaptation to 

 similar modes of life, the marsupials have developed groups parallel 

 to those of the placental mammals. However, it is certain the Quad- 

 rumana seem represented by the Phalangers, the Carnivora by the 

 Dasyuri, Insectivora by the Phascogales, Ruminantia by the kanga- 

 roos, and Edentata by the Monotremes. Rodents and bats are nu- 

 merous in Australia, but only one of the former is marsupial, and none 

 of the latter. The subdivisions of the order are indicated by the 

 modifications of the extremities and digestive system. A gradual 

 transition is found passing from the Phalangers through the Parame- 

 lidae to the kangaroos. All arboreal species have an opposable thumb. 

 This thumb is rudimentary or wanting in the terrestrial species, but 

 in botli the carnivorous and herbivorous groups we find a gradual 

 transition to the species possessing a well-developed thumb; thus the 



