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; 
