3586 The Zoologist — July, 1873. 



thai we may allow infinite scope for variety of plan. Mr. Darwin 

 has referred to the growth and affinities of language in illustration 

 of the growth and affinities of species : it will bring it home to us, 

 and may be not much overstraining the case, if I say that to expect 

 to find the same organization developed from similar external 

 conditions, on Mr. Darwin's hypothesis, will be a parallel case to 

 expecting to find the same language evolved from two originally 

 distinct people who had no communication, because their external 

 conditions were similar. Imagine the European discoverers of 

 Japan finding the natives speaking a European language, or one 

 so near it as readily to be understood by them. Would they not 

 find it very difficult to believe in an independent origin for that 

 language ? Just so, then, we ought to find an absence of placental 

 animals in Australia, if it was separated from the rest of the world 

 before that type was introduced. So far the illustration is entirely 

 against us. 



But let us loot at this order of marsupial Mammalia a little more 

 closely, and see of what divisions it is composed. We have the 

 familiar kangaroos, which take the place in the Australian fauna 

 that the lighter ruminants — antelopes, goats, sheep and deer — do in 

 the larger continents, browsing on the herbage of the plains or 

 amongst the rocks during the dusk, and lying hid in the light day- 

 time. Here is the same place in nature filled by how different 

 and original a type. Then we have the wombat : this animal is 

 to all intents and purposes a rodent; its four front teeth possess a 

 persistent pulp continually growing forwards on the arc of a circle 

 as they wear away in front, with a strong plate of enamel arming 

 their front surfaces, so as to keep this sharp by the greater wear of 

 the softer tissues. Behind, the body of the tooth, consisting of 

 dentine, is surrounded on its surface by a layer ofcemejiium or bone 

 substance. These incisors are separated from the grinders by a 

 wide space unoccupied by any teeth. The same arrangement 

 may be seen by any one who will take the trouble to examine the 

 skull of a rat or guinea pig ; a rabbit or hare differs in having four 

 instead of two upper front teeth. But it is possible that the 

 placental type may, in the first instance, have branched off from 

 a marsupial rodent. Mr. Darwin himself draws attention to the 

 affinity exhibited by the viscacha, a rodent of South America, some- 

 thing like a hare in general appearance. 



I have constructed a genealogical tree of the principal divisions of 



