378 SCOTT. [Vol. V. 



the successive horizons represent a longer time than do the 

 formations themselves. So far, at least, as the fresh-water 

 Tertiaries of North America are concerned, this assumption 

 receives little support from geology, and we may fairly infer 

 that the geographical changes which brought about the drying 

 up of the great lakes in one region and their appearance in 

 another were accompanied by an accelerated rate of change in 

 the organic world. 



The conclusions as to the mode or laws of evolution which 

 thus seem to follow from the study of series of fossil mammals 

 can make no pretensions to novelty, as they have all been 

 repeatedly suggested before. Nor can they be regarded as 

 entirely conclusive, for some of them, as we have already seen, 

 may be fairly interpreted in several ways ; it is not even possi- 

 ble, in the present state of knowledge, to entirely avoid all pre- 

 suppositions in the construction of the phyletic series. But inas- 

 much as there is so little agreement upon these points, I have 

 thought that it would be of service to clearly state just what the 

 problems are and how the known facts seem to be related to 

 them. On some of the questions the evidence speaks with no 

 uncertain sound ; thus, for example, arrange the phyletic series 

 how we may, the occurrence of parallelism and convergence in 

 development is clear. This conclusion can only be avoided by 

 the assumption of complex cross and reticulated relationships, 

 such as no one has ever ventured explicitly to defend. I 

 venture to hope that morphologists may test and extend these 

 tentative results by applying to them other and wider classes of 

 facts than those from which they have been drawn. 



ON SOME OF THE FACTORS IN THE EVOLUTION OF THE 



MAMMALIA. 



The question of evolutionary philosophy as to which there is 

 at present the most dispute is undoubtedly that with regard to 

 the causes of variation, together with -the problems of hered- 

 ity, which are inseparably connected with it. The brilliant 

 essays of Weismann have done great service in calling renewed 

 attention to these fundamental questions, for there can be no 

 doubt that most speculations upon the subject had been taking 



