ADJUSTMENT, DIRECT AND INDIRECT 



curious parallelism. For example there is an order 

 of edentate marsupials, there is a marsupial or- 

 der of carnivora, and another of insectivora, and 

 another of rodents, while the kangaroo strongly 

 resembles the sub-order of ruminants, and the 

 opossum is clearly related to the lemurs, or low- 

 est of the primates. It becomes, then, an inter- 

 esting problem to settle the genetic relationships 

 between the two sub-classes. Did the order of 

 apes descend from the ape-like marsupials, the 

 monodelphian carnivora from the didelphian car- 

 nivora, the higher rodents from the marsupial 

 rodents, and so on ? If so, it is difficult to see 

 how the pouch should have been lost, and the 

 placenta developed in so many different orders 

 independently : such a number of exact coin- 

 cidences seem hardly probable. On the other 

 hand, did all the monodelphia descend from one 

 didelphian form ? If so, it is strange that the 

 differentiation into orders should have gone on 

 so similarly in the two sub-classes, resulting, for 

 example, in the production of marsupial mice 

 which in general appearance are hardly distin- 

 guishable from placental mice. 



Birds and reptiles present an equally puzzling 

 cross-relation. Upon no theory are these the 

 direct ancestors of mammals, although the low- 

 est mammals are both bird-like and reptilian in 

 appearance. The duck-bill, belonging to the 

 mammalian sub-class of ornithodelphia, some- 

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