IO DISTRIBUTION AND DISPERSION OF LEPTINOTARSA. 



Obviously some of these are of more value than others, and some are of no 

 use whatever for our purposes (No. 10). Others are based primarily upon 

 the unproven assumption that evolution has been by certain methods (No. 8). 



The first criterion is easy of application in the present case. Of the 43 

 known species of Leptinotarsa, 3 have been found north of the Rio Grande, 

 6 in the North Mesa, 25 in the South Mesa and the escarpment, 2 1 in northern 

 Central America, and 2 reach as far south as Panama and the Isthmus of 

 Darien. Clearly the area of greatest specific differentiation for the genus is 

 southern Mexico, or the region occupied by the southern end of the Mexican 

 division of the North American region and the northern end of the Central 

 American region. This area, situated in the main between the plateaus of 

 Mexico and Guatemala, is now maturely dissected and immensely diversified. 

 Three-fourths of the species of the genus are found in this region, and of 

 these nearly the entire number are confined entirely thereto. In the lineata 

 group, of the 12 known species 3 are found in the country to the north of 

 the Rio Grande, 1 both north and south, 1 in the North Mesa, 8 in southern 

 Mexico, and 1 in Guatemala and southward to the Isthmus of Darien. In 

 this group the same proportions hold, i. c, three-fourths of the species in 

 the region in which the genus as a whole shows its dominance, with a stronger 

 representation to the north than to the south. By this first criterion the 

 center of origin is clearly shown to be in southern Mexico. 



The second criterion is not as easy of application as the first, largely 

 because the data from many regions are meager or altogether wanting. 

 First, to consider the genus as a whole : North of the Rio Grande, one 

 species, L. dccemlincata, is exceedingly abundant, surpassing all of the other 

 species in the genus in point of numbers of individuals. Dominance in this 

 area is not of the kind that would have significance in the object of our 

 search, because the records show that this species in the region where it is 

 most abundant is an introduced species, a new member of the fauna, and is 

 at present subject to the laws governing such cases. If, however, this fact 

 were not known, if the introduction had taken place 100 or 200 years ago, 

 or even if the introduction had passed unnoticed, we should be obliged to 

 place the dominance of individuals in the Mississippi Valley and eastward. 

 Dominance, or great abundance, of individuals is a point carefully avoided 

 in his notes by the professional collector, the records of the collectors who 

 have worked in Mexico and Central America being silent on this point. 



Excepting in the case of L. decemlineata , my observations indicate a region 

 of great abundance of individuals in southern Mexico. One species, L. unde- 

 cimlineata , appears, however, to be common and widely distributed in 

 Guatemala. Large series have been recorded from there and extensive 

 collections exist in several museums. We conclude in regard to the second 

 criterion that it shows the center of origin of these beetles to be in southern 

 Mexico, but that the evidence derived from this criterion may be seriously 



