210 



The Mechanism of Evolution in Leptinotarsa 



Table 26 shows local diversity in the directions of variation, and they may be, 

 as far as biometrics go, either germinal or somatic. Experimental testing, aided 

 by adequate field observations alone, can decide, and this I have done in this 

 instance by bringing stocks from all of the three local populations to Chicago, 

 where under favorable conditions of culture they thrived and gave a combined 

 result shown in the table 27. 



Table 27. — Results found in stocks from three Cuernavaca District colonies, when 

 reared under like conditions at Chicago. 



It is certain from this test that the three stocks are different in gametic con- 

 stitution ; otherwise why should they retain, under like conditions, when grown 

 side by side, the same differences observed in nature and tested in this simple 

 experiment? The result shows the fixity and specificity of these minute and 

 trivial characters in the organisms, and also the delimited, determinated 

 response in the directions of heterogeneity in the different locations, but to what 

 agency this determinate response is to be attributed is not indicated. In the 

 amount of pigmented area perhaps more certain determinate response is 

 observed, but this is of no significance. 



In L. diversa and its geographic variety rugosa, little need be said. No 

 directions of variation are found, and it has not been observed to be absent in 

 any individual that was able to transmit the character as absent. Spot c (fig. 

 27) is always a round area of variable size and it is always free, never fusing 



Fig. 27. — Showing absence of variation in directions and 

 difference in size found in spot c in L. diversa. 



with any other center ; even in area it is fully as conservative as it is in shape. 

 In the former its greatest change is a condition having an irregular outline. 

 Quantitative determinations made of the area at El Borrego and Sierra de 

 Escamela, near Orizaba, Mexico, and at El Riego, near Tehuacan, Puebla, 

 Mexico, for its variety rugosa, show the constant differences and shape of 

 polygon, represented in figure 28. That their value is of no moment is shown 

 by their lack of permanency ; when stocks are taken to the uniform condition, 

 of the laboratory at Chicago at once all assume the same condition. In this 

 example the differences are, as far as determined, merely unfixed habitudinal 

 somatic fluctuations in quantity of pigment due to external conditions acting 

 upon growth conditions. 



