THE INTREPRETATION OF ISOLATION 49 



place along routes where suitable habitats are nearly continuous, such 

 as river valleys for mesophytic species or interfluvial uplands for 

 xerophytic forms. There is every reason to believe that the post- 

 glacial migration of the deciduous forests into the Middle West fol- 

 lowed the river courses almost exclusively. From these earlier migra- 

 tions along the line of least resistance, a slower type of migration 

 gradually extends the range out upon other habitats. This follows 

 regularly the physiographic development of an area, as Cowles has 

 shown, 21 and may also be induced by the reaction of an association 

 upon the environment in the margin of a contiguous association beyond 

 it. 11 So the forests of the Middle West, entering the region along the 

 nearly continuous habitats of the river courses, where they migrate 

 rapidly, have also spread at right angles to them, but more slowly, until 

 in some cases those of different river systems have become united. 



Each important change in the climate or the configuration of the 

 land, due to geological causes, has created new migration routes and 

 destroyed old ones, interrupting migrations then in process and initiat- 

 ing new ones, which may progress in diiferent directions and be par- 

 ticipated in by plants of different origin. There is evidence of several 

 such migrations in the Middle West, which are discussed in Part IV. 



The Interpretation of Isolation. — Actual geographical isolation 

 of a species, in the ordinary sense, is effective only when an outlying 

 habitat is separated from the general range by a distance greater than 

 the migration-capacity of the plant. Instances of this are too well 

 known to need citation. Only two explanations are possible for such a 

 condition. Either the species has had two separate and independent 

 origins, or else the vicissitudes of migration have brought it about. 

 Even if the possibility of the former alternative is admitted, the latter 

 is the more plausible in the majority of cases. It implies that the two 

 disjunct bodies of the species were at some time one, either in a ter- 

 ritory from which both portions have migrated and which is no longer 

 occupied, or through the existence of connecting links between the two 

 which have now disappeared. Both cases have been caused by a, retreat- 

 ing migration. 



Environmental changes which permit the advance or compel the 

 retreat of one species are quite likely to affect others similarly, since 

 associated species demand in general the same environment. Migra- 

 tion is therefore concerned with numerous species simultaneously. 



21 Cowles, H. C. The physiographic ecology of Chicago and vicinity; a study of 

 the origin, development, and classification of plant societies. Bot. Caz. 31 : 73-108, 

 145-182. 1901. 



