'66 CHAS. C. ADAMS. 



and their influence upon the successional relation of the faunas 

 correlated with the degree of topographic development of a 

 region. Although the fauna was mentioned in particular, these 

 'factors influence the entire biota in a similar manner. During 

 the process of degradation of the land there is a definite and 

 orderly succession of conditions through which the habitats 

 pass ; the brooks become larger streams, the lakes and ponds 

 become drained, and the uplands are lowered, etc. Not only 

 does the location of the habitats change but also their relative 

 positions and extent. On account of the great influence which 

 topographic conditions have upon habitats it is possible to find 

 very diverse biotic conditions even in a relatively small area. 

 Students of local faunas and floras frequently comment upon 

 this diversity, and although these facts are often noted, yet but 

 little attention is given to them because of their seeming chaos. 

 This apparent mixture or confusion is often due to a total disre- 

 gard of the habitats and the associations of the forms in them. 

 That this occurrence is, as a rule, quite definite and orderly, may 

 be seen by reference to the example of certain southeastern 

 types evidently of western or southwestern origin, the yuccas 

 and pocket gophers. These are types from an arid region, and 

 it is important to note that when they invade a moist region they 

 occupy the relatively dry situations the pine barrens, sandy or 

 rocky places--for such are the conditions most nearly approach- 

 ing their original home. Such colonies form "islands" of arid 

 types surrounded by those correlated with greater moisture. The 

 significant fact here is the definiteness of the conditions in which 



o 



they occur. Again this same tendency is shown in the extreme 

 northward extension of the southeastern biota along protected 

 valleys, and even far out upon the Great Plains. Similarly in 

 southern Michigan, certain characteristic members of the south- 

 eastern biota enter the state at the southeastern and southwestern 

 corners, rather than along the southern border, because valley 

 highways enter the state at these corners. Apparently this same 

 route into southeastern Michigan has been utilized by certain 

 forest trees, insects, birds and doubtless other southern types, 

 which have also invaded extreme southwestern Ontario. Such 

 facts might be indefinitely multiplied, but these clearly show that 



