1916] Taylor: Beavers of Western North America 481 



become considerably differentiated from A, and probably will be 

 physiologically isolated from it, and furthermore, will not be under 

 the necessity of competing with A, except in the most general way, 

 for it will be fitted to occupy ecologic niche m, which is different from 

 ecologic niche a. Two species derived from the same original stock 

 will thus be found occupying different ecologic niches in the same 

 place. The closest relatives of each will be found, not in the same 

 locality but in a neighboring area, the races or species being kept 

 separate by a geographic barrier of some sort. Thus B is A's 

 closest relative, and L is M's closest relative. 



It is to be doubted whether an actual series of cases completely 

 illustrative of the steps in such a differentiation and re-invasion can 

 ever be demonstrated in nature. The process is an extremely com- 

 plicated one, as a moment's consideration will show. The diagram 

 is purposely made very simple ; in nature the process must be 

 exceedingly complex. The primitive stock and its descendants are 

 subject to all the vicissitudes of environmental and topographic 

 change, and the process as above outlined would not only require an 

 enormous time for its fulfillment, but would depend upon the 

 existence of spatial relations of exactly the right sort. Possible 

 contingencies in a great many directions are numerous. 



The great weight of evidence from the study of mammals and 

 higher vei-tebrates would seem to indicate that the occupation of 

 different ecologic niches in the same place has in every instance 

 been the result, not of some process of adaptation of a portion of 

 the parent stock to a distinct ecologic niche, and the differentiation 

 of this adapted portion while both were living in the same locality, 

 but of some comprehensive process similar to the hypothetical one 

 above presented, involving migration, differentiation, and re-invasion. 



If, as we believe 'the evidence indicates, geographic isolation is, 

 in the higher vertebrates, a condition essential to specific differenti- 

 ation, then on the assumption that the classes of vertebrates are 

 descended from one or a very few original stocks it follows that 

 these migrations, differentiations and re-invasions have been very 

 extensive. For there are many ecologic niches in almost every 

 locality which are now occupied by various mammals, or birds, or 

 reptiles. It has been long recognized, of course, that there have 

 been numerous migrations and differentiations. But has due recog- 

 nition been accorded the principles of invasion and re-invasion? 

 Unless there had been, according to the logical continuation of the 



