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119 



UINTA SELENODONTS 



with problems of relationship. It was considerations of this kind that first 

 suggested to me the probabiHty that Protoccras must in some way be related 

 to Leptomoyx and its allies, a suggestion which now seems to be very strongly 

 confirmed by the discovery of the new selenodonts from the Uinta beds. 

 Migrations of terrestrial mammals have, of course, repeatedly taken place 

 from one continent to another, as is amply demonstrated by the successive 

 fossil faunas of Europe and North America, but the native stocks are apt to 

 maintain their footing for long periods of time against the invasion of nearly 

 allied forms which play similar parts in nature. It is curious to observe how 

 long a time was required before the Pecora were able to establish themselves 

 in North America. Not until they had reached a very high plane of organiza- 

 tion did they succeed in overcoming the competition of the highly diversified 

 Tylopoda, which had been developed here and which were not entirely driven 

 from this continent until late Pleistocene times. In South America they still 

 continue to flourish, and in that continent the only representatives of the 

 Pecora are the deer in limited variety. 



In cases of doubtful affinities the probabilities are usually in favor ot 

 reference to an indigenous group, for they almost always outnumber the 

 immigrant forms, and it is with such indigenous groups that the most careful 

 comparisons should be made. Neglect of this principle has led most students 

 of the American selenodonts astray. 



These considerations emphasize once more the necessity of obtaining long 

 and fairly complete phylogenetic series for the satisfactory solution of taxonomic 

 problems. Without such series it is exceedingly difficult, often impossible, to 

 escape being deceived by resemblances due to parallel or convergent develop- 

 ment. With these series, such deceptive resemblances may be exposed and 

 allowed only their due weight, while puzzling differences may be followed out 

 in their origin and development and given their proper taxonomic value. 



This paper contains the recantation of many opinions which I formerly 

 maintained concerning the relationships and systematic position of the various 

 groups of peculiar North American selenodonts. I may say, however, that 

 these opinions were not very confidently held, for they never seemed to ex- 

 plain satisfactorily the manifold peculiarities and the isolated position of these 

 families and genera. It may be that the views here expressed are destined to 

 have no longer life, but they do offer a much more satisfactory solution of the 

 problem, and they are at least founded upon a much wider range of evidence 

 than has been available hitherto. 



