212 AFFINITIES AND GEOGKAPHICAL 



% 



The first glimpse of the mammalian type presented by geo- 

 logical research, is in the Cetiosaur, a huge reptile of the 

 Oolite, nearly allied to the marine sauria, but exhibiting in 

 the form of the larger vertebrae a clear affinity to the whale 

 tribes. In connexion with this fact, the physiologist tells us, 

 that in the manatus and dugong, specimens of these tribes, 

 there is a dense texture of bone, and an absence of medullary 

 cavities, assimilating their skeleton to the reptiles, which class 

 they further resemble in the loose connexion of the bones of 

 the head. It would thus appear, in this grand instance, as in 

 all others, that the starting point of the superior class was in 

 the marine and consequently earliest examples of that below ; 

 for the trunk of the genealogical tree of Being is short com- 

 pared with the branches. 



The passage from the Birds to the Mammalia connected 

 with them happens (for accident in the preservation of species 

 and the discovery of fossils is much concerned in the case) to 

 be more manifest. Somewhat unexpectedly but Nature must 

 be taken as we find her it is the humblest mammalian orders, 

 which come through this protracted channel. They are the 

 Digging Edentata (armadillos, ant-eaters, pangolins, etc.), the 

 Insectiwra (moles, hedgehogs, etc.), and Rodentia (mice, 

 squirrels, hares). By all modern comparative anatomists, it 

 is allowed that these show, in their osseous structure, imper- 

 fect dentition, and humble character of brain, an affinity to 

 birds. There is, however, another order of animals, which can 

 hardly be considered as fully mammalian in character ; Im- 

 placental Mammalia they may at the best be called ; and 

 which are even nearer to the bird character than the three 

 orders above enumerated. These are the Monotremes and 

 Marsupials ; animals now almost restricted to Australia, but 

 which were formerly more widely distributed. In the brain 

 and other parts of the organization, they are decidedly bird- 

 like. But this is not all ; for in the only surviving mono- 

 tremes, the Ornithorhynchus and Echidna, we see the bill and 

 web-feet of the swimming-bird still unchanged, the former 

 animal being a semi-rodent, and the latter a semi-insectivore. 

 Here is a gradation most remarkable, the point reached being 

 only, as it were, half-way towards the higher form of existence. 

 It is also very remarkable to find amongst the more numerous 

 marsupials, genera recalling the rodent and the insectivorous 

 forms, as if these were only further advances along similar 

 lines. 



