52 DISTRIBUTION AND DISPERSION OF LEPTINOTARSA. 



THE RELATION OF THE DISTRIBUTION OF THE GENUS LEPTINOTARSA 

 TO NATURAL ENVIRONMENTAL COMPLEXES. 



The geographical distributiou of animals, or animal geography, is usually 

 considered from one of two viewpoints — the static or the d3mamic. Con- 

 sidered from the static standpoint, the facts of distribution are taken and 

 arranged in order according to some empirically chosen standard, and zones, 

 subzones, or other unnatural areas of distribution are established. The 

 study of animal distribution from this standpoint is a dead and profitless 

 pursuit. Dynamically considered, animal geography seeks to explain the 

 facts of animal distribution as w^e now find them in terms of the relation of 

 the animals to each other and to their environmental complexes. It is the 

 dynamic aspect of the distribution of these beetles that I shall consider in 

 the remainder of this paper. 



One of the most striking facts of animal geography is the correlation 

 which exists in any large group of animals between the development of its 

 various species and groups of species and the environmental complexes of 

 the country which forms the habitat of the animals in question, specific 

 differentiation following directly upon changes in the environmental com- 

 plex. This correlation has long been recognized, but whether the relation 

 is cause and effect or onlj^ apparently so, and how the various lines of specific 

 differentiation came into being, are open questions. 



The groups of species in Leptinotaj'sa occup}^ in a marked way one or more 

 of the natural complexes into which the country in which they live is 

 divided. All the groups have their origin in the same area, and from this 

 center have spread out and developed in different lines of species differentia- 

 tion in the different topographic and climatic areas of the country in which 

 they live. 



On examining the table on page 50 it is evident that the two chief areas 

 into which the territory inhabited by these beetles is divided — the Central 

 American and the North American — have almost no species in common. 

 Only one species, L. dahlbofni, can be said to be common to both areas, while 

 another, L. dileda, is found to be an inhabitant of both along their boundary. 

 It is probable, however, that as the number of local records is increased more 

 species will be found to have crossed over from one area to the other and 

 penetrated for short distances inward. This sharp demarkation between 

 the species that are Central American and those that are North American 

 is characteristic of the genus as a whole as well as of each group of species 

 in the genus — a fact that may be seen by an examination of the table. All 

 of the groups excepting rubiginosa and zetterstedti have species to represent 

 them in both areas. 



In order to examine more closely into the distribution of the species accord- 

 ing to topographic areas, let us look for a moment in some detail at the 



