54 CHAS. C. ADAMS. 



tional work has been largely due to the great increase in the 

 amount of data. 



The 'next general step of advance which we may expect to 

 follow this stage of rapid accumulation of facts is that of their 

 explanation or interpretation. At present this phase of the sub- 

 ject is much confused by the babel of opinions as to the relative 

 importance of various influences controlling distribution. There 

 are several points of view, and each worker is keen to the influ- 

 ence of certain factors. It is to be hoped that this diversity of 

 opinion will lead to a period of discussion, enriched by many 

 suggestions and discoveries of relations, previously unnoticed. 

 Such a period would certainly hasten the correlation and inter- 

 pretation of much miscellaneous and imperfectly organized data. 



It is to one phase of the subject of faunal interpretation, and 

 the dynamic aspect of the historic factor in particular, to which 

 special attention is directed in this paper. That the historic factor 

 is a real one is very generally recognized, and yet in spite of this 

 fact it is difficult to fully realize that the present distribution which 

 we see, is largely an effect of past conditions ; the cumulative re- 

 sult of many factors, and not controlled altogether by the condi- 

 tions of the present environment. To properly estimate this 

 factor it becomes necessary to reconstruct the successional rela- 

 tion and the past conditions, and thus see how each stage has 

 prepared the way for the following one. We must reconstruct 

 the past, for this is as essential in geographical distribution as is 

 the restoration, in the mind of the paleontologist, of the soft parts 

 of the fossils he wishes to interpret. 



Some phases of this subject are much more simple than others, 

 just as the history of one region may be much simpler than 

 another. From the biological standpoint this is certainly the 

 case with that part of North America repopulated during the de- 

 cline of the Wisconsin ice sheet. 



To fully understand the return movement to the glaciated 

 region, it is necessary to know the time relations of the various 

 Pleistocene deposits of fossils, as during that time there flourished 

 a variety of forms no longer members of our present fauna. The 

 mastodon, mammoth, peccary, camels, tapirs, native horses and 

 many other forms were then abundant. But, as this phase of 



