No. 3.] MODE OF EVOLUTION- /AT THE MAMMALIA. 36 1 



improbable, and yet when the list of unmistakable parallelisms 

 to be given in the next section is examined, it will appear less 

 unlikely. Nor will the difficulty be diminished by assuming 

 that the resemblances to the Pecora are due to genetic affinity, 

 because then we should have to account for an even greater 

 number of resemblances to the tragulines, which could not have 

 occurred in the ancestor common to this group and the Pecora. 

 If we agree with Riitimeyer, and refer Leptomeryx to the fore- 

 runners of the camels, then the difficulties become all the 

 greater. 



Obviously, the final solution of the question can only be 

 reached when we have learned more of the selenodonts of the 

 Uinta formation, and are enabled to trace the ancestry of 

 Leptomeryx back to the Eocene forms. From the evidence at 

 present available, I cannot doubt that the genus has a real 

 relationship to the tragulines, with certain independently ac- 

 quired resemblances to the Pecora, resemblances which appear 

 to be carried even further in the curious little genus Hypisodus, 

 with its prismatic molars and highly specialized inferior incisor 

 series. 



ON THE MODE OF EVOLUTION IN THE MAMMALIA. 



We may now attempt to apply the results gained from the 

 history of the fossil mammals, discussed in this and the preced- 

 ing paper, to the problems which were stated in the paper on the 

 Tylopoda (No. 52, pp. 2-9). In the discussion which follows 

 I shall not undertake to go over the whole vast field which these 

 questions open up, but shall confine myself as far as possible to 

 the mammals, using other groups of organisms only occasionally 

 as a means of comparison and illustration. To do more than 

 this would require a volume. 



1) The first question propounded, viz. that as to the single or 

 multiple origin of genera, is at bottom rather a question about 

 words than about things, and the answer which we make to it 

 will to a great extent depend upon the view taken as to the 

 definition of the word genus. If we make classification an 

 expression of real relationship, and not of mere similarity of 

 structure, as should certainly be the end proposed, then it is 

 obvious that all the species of a genus must be more nearly allied 



