VIII. ECOLOGY — INTERPRETATION OF 



ENVIRONMENT AS EXEMPLIFIED 



IN THE ORTHOPTERA 



Sources of Life after Glaciation 



AT the time of the glaciation of North America, the 

 lower Lake Michigan region, with which we are 

 here interested, was submerged in an ice sheet. 

 (See map,i page 319.) The lake formerly extended 

 over a much wider area than it does at the present time. All 

 the life now occupying this region has become established 

 here since the decline of the glacial ice sheet. Whence comes 

 the source of life found here at the present time is not fully 

 determined. But much thought has been expended on the 

 general subject of post-glacial dispersal of North American 

 animals and plants by workers in this field. Especially 

 notable are the researches carried on by the United States Bio- 

 logical Survey. Adams ^ and others have given special atten- 

 tion to this most interesting subject, and the reader is referred 

 to their published papers. In general, it may be said that the 

 fauna and flora of the northern United States, east of the Great 

 Plains, are geographically related to those of the southeast. 

 In other words, from the southeastern part of the United 

 States we have derived a great part of our fauna and flora, 

 except in the case of some species which are supposed to have 

 their origin in the north. 



Other species are also known to have come from the arid, 

 southwestern part of the United States. The Mississippi 

 Valley has been an important highway for the dispersal of 

 forms from the southeast. The study of the local flora and 

 fauna over great areas will ultimately throw much light on 



' See Leverett's published map in one of the Bulletins of the Geological 

 and Natural History Survey of Illinois, published . by the Chicago Academy 

 of Science. 



* Biological Bulletin, Vol. IX, 1905, and others. 



317 



