56 DISTRIBUTION AND DISPERSION OF LEPTINOTARSA. 



more favorable habitat, and accidental causes are insufficient for the explana- 

 tion of natural phenomena. A species must possess a certain amount of 

 adaptation to its original habitat in order to live and gain strength to spread 

 out into one to which it is better adapted, as did deccmlincata by migration. 

 In the group of beetles to which decemlineata belongs the development of 

 the present distribution and specific differentiation has certainly been pro- 

 foundly influenced by factors of the environment in which it lived. At 

 least no other conclusion is warranted from the data at our command. 



If we should examine the data concerning the other groups in the genus 

 Leptinotarsa in the same manner as we have the lineata group, a similar set of 

 observations and the same arguments would be given. As far as I am able 

 to discover, they all show the same exact limitation to and dependence upon 

 a particular environmental complex. All of the species of the genus are 

 limited to grassland habitats that is, to areas in which there is a frequent 

 atmospheric precipitation of greater or less amount, which keeps the surface 

 of the ground and the lower stratum of air moist during the summer or 

 growing season and with a distinct dry or winter season. The temperature 

 is of little moment, as long as it is not low enough to inhibit growth processes 

 during the moist season. The essential vegetation is perennial grasses grow- 

 ing in bunches with annual, herbaceous, or woody plants, often of consid- 

 erable size. The environmental complex, which I here term " grasslands," 

 is a natural combination of physiographic, climatic, and plant conditions, 

 which together form an environmental complex outside of which Leptinotarsa 

 can not live. Within this general type of habitat there are, however, exten- 

 sive and often striking differences, and the grasslands are further divided by 

 local or edaphic conditions into grasslands that are physiologically moist and 

 physiologically dry. The physiologically dry areas are differentiated by 

 local or edaphic conditions which differ in individual cases but produce in the 

 end the same general type of environmental complex savanna, steppe, or 

 semi-desert. We recognize, therefore, four chief divisions of the grasslands 

 meadow, savanna, steppe, and semi-desert. Each of these is modified locally 

 by edaphic conditions and between them there are all possible transition 

 stages, and the change from one to the other is never abrupt, although it may 

 be rapid ; there is never any discontinuity. 



Some idea of the distribution of the species of Leptinotarsa in the four 

 chief types of grassland habitats may be gained from the table on page 58. 

 From this table it is evident that the species of Leptinotarsa are limited to 

 the physiologically dry grasslands, only one being found in physiologically 

 moist areas. By far the larger portion of the species are found in the 

 savannas, a few in the steppe, and also in the semi-desert. Although lim 

 ited almost entirely to the savanna type of grassland habitat, it differs 

 greatly in its edaphic factors even in nearby areas, and it is the changes in 



