122 NATURE STUDIES. 



The meaning of this table becomes clear, if it be 

 borne in mind that the rocks as here noted are 

 divided into the older Secondaries and the newer 

 Tertiaries. The Eocene in turn is the oldest (or 

 lowest) series of the Tertiary rocks, as the Trias is 

 the oldest of the Secondary rocks. 



The fossil remains of birds are few and far between, 

 and this for the reason pointed out by Lyell namely, 

 that the body of a bird falling into water, prior to its 

 entombment in the deposits which form the rocks of 

 the future, would float, and would afford a likely 

 object of prey to other animals; thus escaping the 

 chances of preservation. For long, fossil birds were 

 regarded as limited to the Tertiary rocks ; but we now 

 know of their existence in the Chalk, or Cretaceous 

 period ; and we have also obtained fossil specimens 

 from the rocks immediately preceding the Chalk in 

 time, namely the Oolitic or Jurassic Period. 



It is almost needless to remark that the bird- 

 remains of the Tertiary rocks, as a rule, resemble 

 closely the birds of our own day. In this light they 

 only testify to the age of some of our existing groups 

 of birds, and do not directly support the theory of 

 evolution, whilst, of course, they do not in any way 

 negative it. But in the deposits of the London clay 

 of Sheppey, belonging to the Eocene (Tertiary) 

 period of geology, the remains of a bird, belonging 

 apparently to the swimmers, were discovered. This 

 form was named Odontopteryx by Professor Owen, 

 and its remarkable jaw-armature at once attracted the 



