No. i.] OSTEOLOGY OF PCEBROTHERIUM. 9 



and from the germ to the body of the offspring. At present 

 such transmission is neither proved as a fact, nor has its assump- 

 tion been shown to be unquestionably necessary" (pp. 104, 105). 

 " If, as I believe, these phenomena can be explained without the 

 Lamarckian principle, we have no right to assume a form of 

 transmission of which we cannot prove the existence. Only if 

 it could be shown that we cannot now or ever dispense with the 

 principle, should we be justified in accepting it. . . . Only if 

 the phenomena presented by the progress of organic evolution 

 are proved to be inexplicable without the hypothesis of the 

 transmission of acquired characters, shall we be justified in re- 

 taining such an hypothesis" (p. 448). 



Closely akin to this most important problem is the question 

 whether development is always by minute gradations or whether 

 it may not be per saltum. 



The study, detailed and minute as it must be, of mammalian 

 phyla, which can be regarded as at least approximately deter- 

 mined, promises to throw much very welcome light upon these 

 vexed questions ; for we have here, as it were, a great series of 

 physiological experiments carried out, not through a few gen- 

 erations or even centuries, but through long ages. It would 

 certainly seem that an examination of the steps of gradual and 

 progressive change must afford weighty evidence upon one side 

 or the other, and make clear which hypothesis best accords with 

 the observed facts. 



One of the series which will prove the most serviceable for 

 our purpose is undoubtedly that of the Tylopoda, which has been 

 so satisfactorily worked out by Cope (No. 6, p. 341 ; No. 9) 

 both on account of their great numbers, their excellent preser- 

 vation, and their persistence with steadily advancing differen- 

 tiation through long periods of time. Leaving aside for the 

 present the discussion of the mutual relationships of the various 

 genera of this series, we have here merely to enumerate these 

 forms in the order of their appearance in geological time. The 

 earliest member of the series would appear to be the genus 

 Pantolcstes, Cope, of the Wasatch, which is followed by Homa- 

 codon, Marsh, of the Bridger, a genus which is very closely 

 allied to, if not identical with, the European Dichobune, from 

 which Schlosser derives the true ruminants and the tragulines. 

 In the Bridger also occurs Ithygranunodon, O. S. and S., but 



