238 



The Mechanism of Evolution in Leptinotaesa 



The simplest and easiest method is to assume that this condition represents 

 wide fluctuation in the quantity of pigment and that it spreads in all directions, 

 producing the fusions and different patterns found. Upon this basis the essen- 

 tial phenomenon is the amount of pigment and the variation is one of quantity. 

 In my 1906 paper, page 99, the place variation of L. multitcBniata at Guadalupe, 

 Federal District, Mexico, is given for six generations, i. e., 1903, 1904, and 1905, 

 from the biometric standpoint, and is expressed in terms of the area of the prono- 

 tum covered by the dark pigment, no pigment being per cent and entirely black 

 100 per cent. When seriated the successive generations show only that the 

 amount of surface colored black varies from generation to generation ; that the 

 polygon of distribution of these quantities shifts plus and minus with different 

 means and modes and unlike curves. These determinations I might have plot- 

 ted in curves of distribution, but from them no further information can be 

 obtained, and no amoimt of mathematical treatment can extract a single addi- 

 tional fact of interest. All the essential facts of the different patterns pre- 

 sented in these years, the way in which these patterns reacted in the reproduc- 

 tion of the population, and the real relations of the elements composing the 

 pattern are lost in the statistical methods employed. 



In the years 1903 to 1910 observations upon the heterogeneity of several trop- 

 ical species were made, together with the attempt to analyze the findings. In the 

 northern form, L. decemlineata, similar analyses of the variation at Chicago 

 from 1903 to 1907 were made. It was the experience with L. decemlineata from 

 Massachusetts and Chicago that gave the original clue to imdertake a more exten- 

 sive analysis with the more favorable materials of the Mexican tropics. 



L. midtitceniata, as shown in Chapter II, is variable in composition, is limited 

 in distribution to the Mexican Plateau, where its actual habitat is localized, so 

 that little interchange between different colonies takes place. From 1903 to the 

 close of 1910 continuous observations were made upon this species in several 

 locations. By continuous observations I mean study and actual contact with this 

 species in nature at stated periods in each of its two annual generations through- 

 out the entire period. This necessitated either actual residence in the country, 

 as in 1903, 1905, and 1906, or two or more visits to Mexico per year for purposes 

 of observation. A series of from 10 to 16 consecutive generations were thus 

 tested and analyzed in this investigation. 



LOCATIONS FOR OBSERVATION. 



In 1903 a series of locations were selected after a preliminary series of obser- 

 vations had been made, and actual work was begun in 1904. These locations 

 were chosen along the southern edge of the Mexican Plateau, and presented a 

 diversity of conditions. At these locations two sets of problems were studied : 

 (1) the analysis of heterogeneity in the individual in different locations; (3) 

 the analysis of heterogeneity in the population through a period of years. The 

 two are interdependent, and especially is the second dependent upon the first. 

 The locations selected were as follows : 



A. Valley of Mexico : Chapultepec colony, Tlalnepantla colony, Tex- 



coco colony. 



B. Rio Atoyac Valley : Puebla. 



C. Slopes of El Volcan de Citlaltepetl : San Andres Chalcicomula. 



