40 THE GEOLOGICAL EVIDENCES 



the two. From the existing evidence before us 

 we are safe in concluding that the line of descent 

 of these animals is direct from a reptilian stock ; 

 and this being admitted, there is no anomaly in 

 the fact that the mammals appeared before the 

 birds. Both birds and mammals are divergent 

 modifications from a common axial stock. It 

 is certainly an interesting feature bearing upon 

 the reptilian relationship of the Mammalia that 

 the earliest reptilian forms, those of the Permian 

 period, are the only animals which possess the 

 remarkable dental characters of the Mammalia. 

 These, as is well known, have the teeth di- 

 vided into three series incisors, canines, and 

 molars a structure unknown among other living 

 animals. But in the reptiles of the Permian 

 period, which may perhaps be looked upon as 

 the ancestral stock whence the Mammalia were 

 derived, the same dental feature is presented. 



