FOUND LINKS. 121 



" air sacs " in tlie bird's body, and also fills the 

 interior of the bones in most birds. Such a dis- 

 tribution of air in the bird's body is evidently 

 adapted for the exigencies of flight. .On the whole, 

 then, with certain well-marked likenesses which, bo 

 it observed, evolution accounts for on the idea of a 

 common origin the classes of birds and reptiles are 

 demarcated from one another by certain highly- 

 distinctive characters. 



The dissimilarities on the hypothesis of evolution 

 are due to variation and modification ; but, if this idea 

 be correct, can we show the stages through which the 

 variation had led these two groups ? In other words, 

 have the ' ' links " which should hypothetically connect 

 them, any existence whatever ? Such an inquiry 

 would have been answered in the negative only a few- 

 years ago; but, thanks to recent research, we are 

 now enabled, satisfactorily enough, to bridge the gulf 

 between birds and reptiles, and in a measure to 

 reconstruct the pedigree of these curious races. 



To render my remarks clear, ifc may be well at this 

 stage to show in a tabular form the relative positions 

 of the rock-formations with which we shall have to 

 deal. Placed in the order in which they occur in the- 

 earth's crust, the rocks in question lie thus : 



f Eecent. 



Tertiary ) Pliocene. 

 Hocks. } Miocene. 

 Eocene. 



Trias. 



