GEOGRAPHICAL VARIATION. 



I05 



The following laws have been found to hold concerning place variation, as 

 far as the present investigation goes : 



(1) Place variation is a universal phenomenon, but it varies in its extent, 

 being greatest in species of wide distribution and high individual variability 

 and least in species of restricted habitat and low variability. 



(2) Place variation is productive of prophetic and historic skewness in 

 the polygone of distribution. (Unless this factor be accounted for in skew 

 polygons we may safely attribute skewness to this cause.) 



(3) The extent of place variation shown by a species is a good index of 

 its susceptibility to external stimuli. 



(4) In place variation, whenever there occurs an extreme oscillation of the 

 population there is an accompanying production of an unusually large per- 

 centage of extreme variations or mutants. 



GEOGRAPHICAL VARIATION. 



Variation in general color, or a tendency toward melanism or albinism, is 

 a common and long-recognized phenomenon, and one exhibited by almost all 

 wide-ranging genera. This variation has been attributed to changes in 

 environmental factors, such as temperature and moisture, to latitude or longi- 

 tude, to altitude, and more rarely to topography and other natural conditions. 

 In Leptinotarsa abundant variation in the general hue is found from place to 

 place, which may be attributed to moisture, temperature, food, sunshine, soil, 

 and other natural causes. There at least exists a striking correspondence 

 between the variation of these beetles and the changes in their environment. 



In the study of the geographical variation of this genus the statements of 

 the conditions found in different places have in every case been based upon a 

 number of generations taken from one locality. In this way the error which 

 might otherwise be produced by place variation is, I believe, to a considerable 

 extent eliminated. 



The data of geographical variation as it has been worked out is most easily 

 presented in tables, in which 20 equals total melanism and o total albinism. 



Tabi.e 39 Geographical variation in the general color of Leptinotarsa. 



