CH. xii.] ADJUSTMENT, DIRECT AND INDIRECT. 61 



the genetic relationships hetween the two sub-classes. Did 

 the order of apes descend from the ape-like marsupials, the 

 monodelphian carnivora from the didelphian carnivora, 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 coincidences 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 

 distinguishable from placental mice. 



Birds and reptiles present an equally puzzling cross- 

 relation. Upon no theory are these the direct ancestors of 

 mammals, although the lowest mammals are both bird-like 

 md reptilian in appearance. The duck-bill, belonging to the 

 mammalian sub-class of ornithodelphia, somewhat resembles 

 a lizard with a bird's beak. Embryology shows that the 

 three classes are divergent offshoots from an amphibious 

 or batrachioid ancestor ; but the birds and reptiles resemble 

 each other much more closely than either resembles the 

 mammalia, so that Prof. Huxley joins them together in the 

 Buper-class or province of sauroids. So far all is plain ; but 

 when we inquire by what forms the birds and reptiles are 

 linked most closely together, we are met by a difficulty. 

 Birds are divided into two sub-classes: tlie ostrich, cassowary, 

 emeu, dinornis, etc., are grouped together as struthious birds, 

 while all other existing forms belong to the sub-class of 

 carinate birds. Now until quite lately it was supposed that 

 ill birds were descended from an extinct reptilian form like 

 "hat ancient reptile, the flying pterodactyl. For the resem- 

 blances in structure between the pterodactyls and the carinate 

 birds are striking enough to have suggested an immediate com- 

 aaunity of origin. Nevertheless, within the past seven years, 



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