196 The Palaeontological Record. I. Animals 



The origin of the mammalia, as a class, offers a problem of which 

 palaeontology can as yet present no definitive solution. Many 

 morphologists regard the early amphibia as the ancestral group from 

 which the mammals were derived, while most palaeontologists believe 

 that the mammals are descended from the reptiles. The most ancient 

 known mammals, those from the upper Triassic of Europe and North 

 America, are so extremely rare and so very imperfectly known, that 

 they give little help in determining the descent of the class, but, on 

 the other hand, certain reptilian orders of the Permian period, 

 especially well represented in South Africa, display so many and such 

 close approximations to mammalian structure, as strongly to suggest 

 a genetic relationship. It is difficult to believe that all those like- 

 nesses should have been independently acquired and are without 

 phylogenetic significance. 



Birds are comparatively rare as fossils and we should therefore 

 look in vain among them for any such long and closely knit series as 

 the mammals display in abundance. Nevertheless, a few extremely 

 fortunate discoveries have made it practically certain that birds are 

 descended from reptiles, of which they represent a highly specialised 

 branch. The most ancient representative of this class is the extra- 

 ordinary genus Archaeopteryx from the upper Jurassic of Bavaria, 

 which, though an unmistakable bird, retains so many reptilian 

 structures and characteristics as to make its derivation plain. Not 

 to linger over anatomical minutiae, it may suffice to mention the 

 absence of a horny beak, which is replaced by numerous true teeth, 

 and the long lizard-like tail, which is made up of numerous distinct 

 vertebrae, each with a pair of quill-like feathers attached to it. Birds 

 with teeth are also found in the Cretaceous, though in most other 

 respects the birds of that period had attained a substantially modern 

 structure. Concerning the interrelations of the various orders and 

 families of birds, palaeontology has as yet little to tell us. 



The life of the Mesozoic era was characterised by an astonishing 

 number and variety of reptiles, which were adapted to every mode of 

 life, and dominated the air, the sea and the land, and many of which 

 were of colossal proportions. Owing to the conditions of preserva- 

 tion which obtained during the Mesozoic period, the history of the 

 reptiles is a broken and interrupted one, so that we can make out 

 many short series, rather than any one of considerable length. 

 While the relations of several reptilian orders can be satisfactorily 

 determined, others still baffle us entirely, making their first known 

 appearance in a fully differentiated state. We can trace the descent 

 of the sea-dragons, the Ichthyosaurs and Plesiosaurs, from terrestrial 

 ancestors, but the most ancient turtles yet discovered show us no 

 closer approximation to any other order than do the recent turtles ; 



