XIII 
THE GEOLOGICAL EVIDENCES OF EVOLUTION 
407 
Birds are among the rarest of fossils, due, no doubt, to their 
aerial habits removing them from the ordinary dangers of 
Hood, bog, or ice which overwhelm mammals and reptiles, and 
also to their small specific gravity which keeps them floating 
on the surface of water till devoured. Their remains were long 
confined to Tertiary deposits, where many living genera and 
a few extinct forms have been found. The only birds yet 
known from the older rocks are the toothed birds (Odontor- 
nithes) of the Cretaceous beds of the United States, belong¬ 
ing to two distinct families and many genera; a penguin-like 
form (Enaliornis) from the Upper Greensand of Cambridge ; 
and the well-known long-tailed Archaeopteryx from the Upper 
Oolite of Bavaria. The record is thus imperfect and fragment¬ 
ary in the extreme ; but it yet shows us, in the few birds dis¬ 
covered in the older rocks, more primitive and generalised 
types, while the Tertiary birds had already become specialised 
like those living, and had lost both the teeth and the long 
vertebral tail, which indicate reptilian affinities in the earlier 
ages. 
Mammalia have been found, as already stated, as far back 
as the Trias formation, in Europe in the United States and 
in South Africa, all being very small, and belonging either 
to the Marsupial order, or to some still lower and more 
generalised type, out of which both Marsupials and Insectivora 
were developed. Other allied forms have been found in the 
Lower and Upper Oolite both of Europe and the United States. 
But there is then a great gap in the whole Cretaceous 
formation, from which no mammal has been obtained, although 
both in the Wealden and the Upper Chalk in Europe, and in 
the Upper Cretaceous deposits of the United States an 
abundant and well-preserved terrestrial flora has been dis¬ 
covered. Why no mammals have left their remains here it is 
impossible to say. We can only suppose that the limited 
areas in which land plants have been so abundantly preserved, 
did not present the conditions which are needed for the fossil- 
isation and preservation of mammalian remains. 
Alien we come to the Tertiary formation, we find mammals 
in abundance ; but a wonderful change has taken place. The 
obscure early types have disappeared, and we discover in their 
place a whole series of forms belonging to existing orders, 
