300 A NATURALIST IN THE GREAT LAKES REGION 



invading forms affect man's crops or his domestic animals. 

 When about the middle of the last century settlers pushing west- 

 ward brought the fields of cultivated potatoes to the edge of the 

 habitat of the Colorado potato beetle, there feeding on wild 

 species of the potato family, the eastward movement of this 

 animal began. The progress of its invasion and the routes 

 followed can be seen on the accompanying map (Fig. 449). 

 Similarly the cotton-boll weevil, which came out of Mexico to 

 invade the United States, is rapidly spreading all over the South 

 where cotton is a crop. Its advance is shown in the accompany- 

 ing map (Fig. 450). Jimson weed (a shortening of the original 

 name, Jamestown weed) was introduced from the Old World into 

 this country at Jamestown in colonial days, from whence it has 

 spread all over the Eastern United States. Russian thistle, 

 introduced into this country by immigrants in wheat seed planted 

 in North Dakota in 1873, has since then spread pretty much 

 all over this country and much of Canada east of the Rockies. 

 This insurgency of living things, impelling them to seek new 

 fields to conquer, soon repopulated, from the three centers named, 

 the territory laid waste by the glacier. Southeastern United 

 'States seems to be the distribution center from which have come 

 most of our deciduous forest trees, the river clams, such snails 

 as the Pleuroceridae and Viviparidae, most crayfish, and a large 

 share of our fish. Undoubtedly many other forms have also 

 come from this same center concerning whose movements we 

 have little or no data. This southeastern center is particularly 

 rich in species and varieties of the forms mentioned. They there 

 attain their maximum size. From this center the connecting 

 rivers and their valleys afford easy highways of invasion. All 

 of which are good criteria for determining centers of immigra- 

 tion. A map of North America on which there is drawn in 

 outline the present range of a number of our important trees 

 shows at a glance that this southeastern area is a center for all 

 (Fig. 451). There are omitted from this the evergreens and 

 such deciduous trees as evidently belong to the northern forest. 



