152 THE POPULAR SCIENCE MONTHLY. 



ingly rare in other parts of the world, the kangaroos and almost all 

 of the great variety of animals of Australia belong to this group. 

 Thus it appears they are mostly tropical. 



The earliest fossil mammals known appear to be marsupials allied 

 to the opossum. In the bone-caverns of Brazil quantities of bones of 

 opossums, such as live in that country now or similar, are found. One 

 species of Dklelphys was found fossil in the Paris Basin, of Eocene 

 formation. Other relatives of the opossum have been found in a fos- 

 sil state, associated with the palaeotherium, anoplotherium, and. other 

 extinct pachydermous quadrupeds ; but the most remarkable are found 

 in Jurassic rocks, as the earliest fossil mammals known. Their dis- 

 covery in this ancient reptilian age in the limestone of Stonesfield was 

 so extraordinary that attempts were made, ou the one hand, to prove 

 that their remains were reptilian ; on the other, to prove that the rocks 

 were of Tertiary origin ; but it has been established, beyond all doubt, 

 that these animals originated in this early reptilian age, and, proba- 

 bly, by descent, either directly or indirectly, from not very remote 

 reptilian ancestry. This relationship is indicated, not only by the 

 fossil remains of marsupials, but also by the anatomical and embry- 

 onic characters of marsupials and monotremes, so far as known. The 

 organization of marsupials seems to be a kind of reptilian and mam- 

 malian combination, as has been shown by the valuable investigations 

 of Prof. Owen, Dr. Coues, and others. 



The monotremes present the lowest grade of mammalian organi- 

 zation, in many respects approaching closely to the oviparous classes 

 of birds and reptiles. It is probably through these that the marsu- 

 pials have gained some reptilian characters. The opossum, for exam- 

 ple, has " a genuine reptilian skull," as Dr. Coues has remarked in his 

 estimable memoir on the anatomy of this animal. 



The main difficulty in tracing out the genealogy of marsupials is 

 that our knowledge of them is confined chiefly to the living forms, 

 while these must be but a small remnant of the whole group as it ex- 

 isted in ancient times, when its members inhabited every land on the 

 face of our globe. Even in the imagination we cannot resurrect the 

 manifold varieties of the past. But, in all probability. Prof. Haeckel 

 is right in believing tliat this group affords a series of forms connect- 

 ing the lower apes or lemuroids above them with the monotremes be- 

 low. This would bring some of the marsupials within the lineage of 

 human ancestry, and, before all others, the opossums seem most closely 

 allied to the lemuroid apes. Indeed, they have already been grouped 

 with man and the apes, although their structure hardly warrants such 

 a classification. Storr congregated into one group all mammals with 

 an opposable thumb. Also, Ogilby adopted the name chciropeds for 

 the same group, and subdivided it into Bimana (men), Quadruinuiia 

 (monkeys), and Pedimana (Semiadse and opossums). 



The characters of groups ai-e generally arranged into categories 



