xiit 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 

 flood, 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 Archceopteryx 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, Avhile 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 Avhole 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. 



When 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, 



