ELEMENTS OF VERTEBRATE PHYLOGENY 139 



in Australia where they had no competition from the higher mammals; 

 and the placentals, which are the familiar hairy, milk-secreting animals 

 of the world and the group to which man himself belongs. 



As one would expect, the birds, the monotremes, and even the mar- 

 supials have quite a bit in common anatomically with the reptiles. But 

 the placental mammals are quite distinct — more different from the mono- 

 tremes than the latter are from the reptiles. This is especially true as 

 regards the eye; and from ocular and other considerations Franz has 

 postulated that the placental mammals originated, not from lower mam- 

 mals, or (Huxley's view) independently from reptiles, but from forms 

 intermediate between the amphibians and the reptiles. There is however 

 no palxontological justification for such a view. The reptiles and birds 

 are so closely related that they are commonly lumped together as the 

 'Sauropsida'; and monotreme eyes — to some extent also marsupial eyes 

 — are sauropsidan in plan except for a radical simplification of the 

 mechanism of accommodation. The eye of the placental mammal is more 

 like that of an amphibian than like that of a reptile, but this is no proof 

 that the placental mammals originated more or less directly from am- 

 phibians. A more likely view is that the placental mammals had an early 

 history of strict nocturnality, during which they depended largely upon 

 other senses and simplified the eye far below the level of complexity of 

 the eye of the reptilian ancestor. The placental eye thus came to simulate 

 the amphibian eye through what might be described as a reversal of 

 evolution. 



For our purposes the placental mammals may be roughly divided into 

 'lower' and 'higher' orders — the former including the insectivores, pri- 

 mates (including man), bats, sloths, armadillos, ant-eaters, and the 'fly- 

 ing lemurs' (Galeopithecus and Galeopterus) ; and the latter comprising 

 the carnivores, seals, whales, and hoofed animals (including the elephants, 

 hippos, etc.). The rodents and lagomorphs may be assigned to the top 

 of the lower series or to the bottom of the higher, depending on one's 

 point of view. 



The tree-shrew and the aye-aye are thus at the bottom of the group 

 and the elk and tiger are at the top — with man very close to the bottom 

 biologically, ranking high only psychologically, as regards his brain and 

 mind. Man's order, the Primates, split away from the Insectivora about 

 50,000,000 years ago. Most of the living groups of mammals have come 

 into existence since that time. Man himself came along only yesterday, 

 but his stock is older than most of the mammalian stocks around him. 



