396 



GENERAL PRINCIPLES 



be misinterpreted. Of two forms, the one which appeared later 

 is not necessarily descended from the other, since both may 

 have arisen independently from still earlier forms. And the 

 one which appeared latest is not necessarily the highest. Birds 

 are as highly specialized as Mammals. But the mammahan 

 type of specialization may be described as a more successful 

 one, and [Mammals are, therefore, usually placed above Birds. 



805. The geological record is very fragmentary and only 

 occasionally do we get a connected story. The history of the 

 snail, Paludina, has been worked out in detail, and we have an 



Fig. 247. — Fossil remains of Archaeopteryx lithographica. 

 (From Galloway, after Claus.) 



account of the changes through which the genus passed during 

 a considerable interval of time. The shell, which was at first 

 very simple, with smooth, rounded contours, became step by 

 step angular and ribbed. So that the later species have httle 

 resemblance to the earlier forms. 



806. The first bird of which we have any record is the 

 Archaeopteryx. This bird had a long tail consisting of a series 

 of vertebrae, fringed on either side with feathers. The wings 

 were provided with three free digits armed with claws. The 

 head was very large and had heavy jaws, both of which were 



