REI.ATION TO ENVIRONMENTAI^ COMPI^^XEJS. 53 



Central American area. Although many of the species present are common 

 to two of the three areas into which the Central American region is naturally- 

 divided, only three, L. dahlbomi, undccimlineata, and libatrix, are distributed 

 throughout. The number of endemic species in each of the different areas 

 is of considerable interest. Thus the oldest land area from the standpoint 

 of the geologist, the Guatemala- Chiapas Plateau, has but two endemic species, 

 L. belti and evanescens ; the next oldest, the Oaxaca- Guerrero highlands, 

 has two, L. lacerata and novemlineata, while the j^oungest of all, the Atlantic 

 slope and lowlands, has four, L. kogei, violesceiis^ pudica, and chalcospila. 



In the North American area the distribution of these beetles shows a 

 similar series of facts. There is no one species that is common to the entire 

 area, and although the escarpment and the south mesa have four specimens 

 in common — L. oblongata, inultitceriata^ dahlbomi^ and dilcda — the different 

 subdivisions have, in general, but few species in common. This area is rich 

 in endemic forms, there being four on the escarpment, six on the South 

 Mesa, three on the North Mesa, one on the Great Plains, and one in the 

 Atlantic Coastal Plain. 



From whatever standpoint w^e examine the distribution of these beetles, 

 whether of large or restricted areas of the country, it is evident that the 

 groups of species in the genus Leptijiotarsa and the species themselves are 

 confined to particular habitats, which habitats are natural topographic and 

 climatic areas or combinations or subdivisions thereof, and that the various 

 groups and species are in some way very closely correlated with the physical 

 conditions of their habitat. Similar correlations are known to exist in many 

 other groups of animals, but not always to the extent found in this genus. 

 The question is how this distribution and the limitation of the various species 

 to climatic or topographic areas came to exist as we now find it. Were the 

 various species produced as the genus was dispersed from its original center 

 by the action of the environment upon a plastic race so that each group and 

 each species represents the effect of some particular environmental complex 

 encountered? Or were the species produced essentiallj^ as we now find them 

 in some ancestral habitat, and from this disseminated, each seeking uncon- 

 sciously the habitat best adapted to its particular needs and, having found 

 it, establishing itself therein ? Can we account for the present distribution 

 on the hypothesis of direct modification in response to environmental stimuli 

 accompanied by natural selection? or must we invoke the aid of unknown 

 factors which will operate to produce species having structures and consti- 

 tutions adapted to various remote environmental complexes, and then, by 

 chance dissemination, have the newly modified organisms and the appro- 

 priate habitats brought together, with the final result that the entire genus 

 shall present a uniform and perfect correlation and adaptation to its envi- 

 ronment? Are the phenomena of animal geography to be explained on 



