300 OUTLINES OF EVOLUTIONARY BIOLOGY 



grounds, and Professor Huxley included both birds and reptiles 

 in one large group, the Sauropsida. The structure of their 

 wings shows, however, that birds cannot have arisen from 

 pterodactyls but must have sprung from some less specialized 

 form, mainly through the development of a dermal exoskeleton 

 in the form of feathers, which constitutes the chief distinguishing 

 feature of the group. It is, therefore, quite in accordance with 

 expectation that the earliest known bird, Archaeopteryx (Fig. 151), 

 should have been found in the lithographic slates of Solenhofen 

 in Bavaria, which are of Upper Jurassic age, and should exhibit 

 characters intermediate between those of existing birds and 

 reptiles. We shall have occasion to refer to this remarkable 

 connecting link more in detail in our next chapter. 

 - The Mammalia, so far as existing evidence indicates, appeared 

 at an earlier date than the birds, but the earliest known mammals 

 were far less highly specialized forms than birds, and although 

 what appear to be mammalian remains have been found as low 

 down as the Trias, the group did not attain much importance 

 i before the Tertiary epoch, when it replaced the reptiles as the 

 dominant group of vertebrates. Opinions have been a good deal 

 divided as to whether the mammals arose directly from amphibian 

 ancestors or from some primitive group of reptiles. On the 

 whole the evidence seems to be decidedly in favour of the latter 

 view, which fits in vpvy^wftll with the first appearance of the 

 mammals just when the reptiles were beginning to become 

 dominant, and with the fact that, as we have already seen, 

 some of the earlier reptiles were extraordinarily mammalian 

 in aspect. 



The earliest mammals were small forms of doubtful affinities. 

 Dr. Smith Woodward remarks in this connection : " It is as yet 

 impossible to determine at what particular stage in the evolution 

 of the vertebrate skeleton the lung-breathers first acquired the 

 characteristic mammalian circulatory system, the milk-producing 

 glands, and a dermal covering of hair," these being the principal 

 features, apart from the skeleton, which distinguish mammals 

 from reptiles at the present day. The difficulty is increased by 

 the fact that some of the anoinodont reptiles had acquired a 

 dentition very similar to that of primitive mammals, while the 

 earliest known mammals are represented by little more than a 

 few fossil jaws and teeth. 



The Mammalia, both living and extinct, may be 'divided into 



