DISPERSAL OF NORTH AMERICAN BIOTA. 6/ 



invading elements tend to enter a region, not only at a definite 

 place, but also tend to remain in definite habitat associations and 

 conditions even after having once entered a region. This habitat 

 individuality causes more or less isolation of the various ele- 

 ments invading a region and furnishes an index to their direction 

 of origin, and at the same time reinforces the idea of the regu- 

 larity of their field relations. To be sure this definiteness be- 

 comes more or less blurred and indefinite along tension lines, but 

 it is not confusing when considered with the proper perspective. 



It is quite evident that the kinds of biota frequenting similar 

 habitats must be largely different in distinct biotic regions. This 

 may be seen by a comparison of the same habitats in regions 

 occupied by distinct biotic types. Thus if a comparison is made 

 between the shell life frequenting the margin of an isolated pond 

 in Michigan and that occupying the similar habitat of a sink hole 

 pond in east Tennessee, a marked dissimilarity is noticed. In 

 the northern pond there will be an abundance of shells belonging 

 to the genera Liuimca and Physa, while in the southern one these 

 genera will be poorly represented or entirely absent. If a similar 

 comparison is made between the shells found in rapidly flowing 

 brooks, from the same regions, the southern stream will abound 

 in shells of the family Pleuroceridae, a family poorly represented 

 in the north. The Limnaeidae are northern in their distribution, 

 and the Pleuroceridse are characteristically southeastern. The 

 same general relations hold for the vegetation ; in the southeast 

 there is the deciduous forest and in the northeast a coniferous 

 one. 



On account of the unique character of the life occupying the 

 same kind of habitats in distinct biotic regions, there results, in 

 these regions, a different succession of forms attending changes 

 in the topography, climate or any other factors which may influ- 

 ence habitats. Thus t/ie succession of ecological associations is 

 likely to be similar, even in very distinct regions when similar 

 processes and conditions are at work, yet the biotic components, 

 the families, genera or species, etc. , are likely to be quite differ- 

 ent. Such relations as these mean that from very diverse kinds 

 of life there tends to be formed, de imu or by association, certain 

 ecological types which become correlated with certain habitats. 



