CHAPTER XVIII 



MODES OF MAMMALIAN EVOLUTION 



Throughout this book the theory of evolution has been 

 taken for granted, as it seemed superfluous to present an out- 

 line of the evidence upon which that theory rests. ''Descent 

 with modification " is now accepted among naturalists with 

 almost complete unanimity, but, unfortunately or otherwise, 

 this general agreement does not extend beyond the point of 

 believing that the present organic world has arisen by descent 

 from simpler and simpler forms. The application of the theory 

 to concrete cases is beset with grave difficulties and gives rise 

 to the most divergent views. The uninitiated reader who 

 takes up a treatise upon some animal group may well be sur- 

 prised to see the apparently minute accuracy with which the 

 genealogy of the series is set forth and the complex relation- 

 ships of its members marshalled in orderly array. Another 

 treatise on the same subject, however, while agreeing perfectly 

 with the first as to the facts, will contradict its conclusions 

 in almost every particular. Indeed, so notorious did this 

 become, that ''phylogenetic trees" were rather a laughing- 

 stock, and most naturalists lost interest in the problems of 

 phylogeny and turned to fields that seemed more promising. 



To some extent, this almost hopeless divergence is inherent 

 in the very nature of the problem, which deals with the value 

 of evidence and the balancing of probabiHties, as to which men 

 must be expected to differ ; but there is another and more potent 

 cause of the discrepancy. When the contradictory schemes are 

 analyzed, it is seen that each is founded upon certain assump- 

 tions regarding the evolutionary process, assumptions which 



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