24 DISTRIBUTION AND DISPERSION OF LEPTINOTARSA. 



and New Mexico in the winter and north in the summer, and further, that 

 the area throughout which this plant was found before it began to spread 

 eastward was coincident with the range of the Great Plains herds of bison 

 at the end of the eighteenth century. Now, while I have often seen the 

 burs of this plant clinging to the hair of cattle, I have not seen them on 

 bison and know of no record of it ; but if they will cling tenaciously to the 

 hair of the cattle of the Great Plains to-day, there is no reason to believe 

 that the thick, curly hair of the bison would form a hold any less secure. 

 On the contrary, the bison would be a far more efficient agent of dissemina- 

 tion because of the ease with which the seed pods would become firmly 

 entangled in its hair and be transported perhaps to great distances. 



We know from actual records that this plant has advanced steadily east- 

 ward in the last fifty years, chiefly by the means of transportation described 

 above, and if this has been its method of dispersal in recent years, in the 

 absence of evidence that conditions have changed we must regard the same 

 method as the essential one in the time before records were made. 



Wherever the food plant goes the beetles go also, provided the soil and 

 climate are favorable, and over the area now occupied by Solatium rostratum 

 conditions for the existence of L. intermedia were favorable. Into this 

 region these beetles must have spread gradually, and on the eastern slope of 

 the Rocky Mountains have undergone modification which resulted in the 

 production of L. decemlineata. The original distribution of dccemlineata 

 was on the eastern slope of the Rocky Mountains northward to the Canadian 

 boundary, eastward into western Kansas and Nebraska, and southward into 

 Texas and New Mexico. In this habitat it was found by Say in 1823. 

 Then, as now, it was probably sparsely distributed over the area, feeding 

 upon Solatium rostratum. I know of no record of its having been found at 

 an elevation of over 8,000 feet, or in the Great Basin. 



In this habitat it remained in stability until 1845 or 1850, when the western 

 extension of human colonization introduced into its habitat a new factor by 

 the addition to the flora of a new plant, Solan?im tuberosum, which proved 

 an acceptable food. 



About 1845 or 1850 the settlers in the Mississippi Valley, in making their 

 advance westward, brought the cultivated potato into the edge of the habitat 

 of dccemlineata, where the beetle soon learned to use it as a food. This 

 extension of the area wherein S. tuberosum was grown into the habitat of 

 dccetnlincata resulted in the removal of a previously existing barrier to 

 further eastward dispersion. This barrier was the wide stretch of country 

 in which no food plant had hitherto been available and into which it could 

 not go without food. The advent of this new food, however, completely 

 removed the barrier, and there lay open to the eastward an expanse of 

 territory where optimum conditions of existence were developed. Into this 



