ii8 COMPARATIVE ANATOMY vin 



These are plainly adaptive characters, and whatever process 

 has produced them in the one case is just as likely to have 

 produced them in the other I might almost say, must have 

 produced them in the other. So far, however, are the teeth of 

 the thylacine and the dog from being essentially " identical," 



FIG. 6. Skull of Thylacine. 



that in numerous non-adaptive and non-functional characters, 

 as the number of the incisors, premolars, and molars, the mode 

 of succession, and the minute structure (all which will be 

 spoken of at a later period of the course), they widely differ, the 

 dog conforming in all these points with the ordinary placental 



FIG. 7. Skull of Shepherd's Dog. 



mammals, the thylacine with the remaining marsupials. Such 

 characters, underlying, in large groups of animals, the various 

 modifications in relation to use, cannot be too diligently sought 

 out as the true landmarks to guide our steps through the 

 intricacies that beset our path in tracing affinities. 



