40 DISTRIBUTION AND DISPERSION OF I^EPTINOTARSA. 



The center of dispersal for L. decemlineata was on the eastern slope of the 

 Rocky Mountain Plateau in Colorado, Nebraska, and Kansas, between the 

 altitudes of 7,000 and 3,000 feet. The dispersal was started by the intro- 

 duction of a single species of plant into the habitat of decemlineata^ which, 

 taken as a food, furnished a base of supplies in the country to the east. 

 This opportunity was seized, and dece7nli7ieata advanced steadily eastward, 

 increasing its numbers and extending its habitat. The line of advancement 

 from its original habitat was at first narrow, being dependent upon the 

 earliest westward extension of civilization along the Platte River Valley and 

 through northern Kansas. 



Having passed its original bounds, the whole eastern half of North America 

 lay open to it. Which way should it go? What factors should determine 

 the direction of its advance ? Two highways were already developed — one 

 leading directly in the route of emigrant trains, stage lines, and settlers' 

 farms, along the North Platte River to eastern Nebraska and Iowa ; another, 

 less important, along the route followed by certain stage lines from Atchison, 

 Kansas, to Denver, Colorado. That the beetle probably followed both of 

 these lines of travel and settlement is indicated by its simultaneous appear- 

 ance at both termini. These lines of dissemination were dependent upon 

 the fact that it was along the routes of travel that settlements had been made 

 and a series of bases of food supply had been created. 



However, about i860, after they had crossed the Missouri River, a more 

 thickly settled portion of the country was encountered, with wider distribu- 

 tion of their new food plant, which removed all further necessity of following 

 paths of human travel. The beetles were now free to respond naturally to 

 the control of the climatic and topographic features of the country into 

 which they were migrating. Throughout the remainder of this dissemina- 

 tion we find them following well-defined trends of movement, frequently 

 along paths or natural highways where human movements are also active. 

 They have not, however, followed these because of the presence of man ; 

 rather, both have followed the same highway because topographic conditions 

 and climatic causes have determined for all organisms the highways of 

 movement. 



The chief trends of movement found are as follows : (i) Northward, up 

 the Mississippi River Valley ; (2) northeastward, along the Great Lakes-St. 

 I^awrence River Valley ; (3) southward, down the I^ake Champlain and 

 Hudson River valleys ; (4) southeast and east, along the Missouri and Ohio 

 River valleys ; (5) seven distinct movements along natural highways from 

 the western region into and through the Appalachian highlands to the 

 Atlantic coast ; (6) northward and southward along the Atlantic coast, and 

 (7) southward along the Mississippi River Valley (plate 9). As the result 

 of transportation by man, the following centers of dissemination were estab- 



