310 The Mechanism of Evolution in Leptinotahsa 



In the season of 1905 the first census showed no change in the character of the 

 population ; if anything it was more completely limited to the central mass and 

 almost entirely in biotype 7. The second determination showed increased 

 ranges within the general mass of the population, and also the presence of 

 marked, divergent groups that were well separated from the rest of the popula- 

 tion. In the males there was a strong group of biotype 5, and in the females 

 a strong one of 14 and a minor one of 15. The records of the two censuses are 

 shown in figures 115 and 116. The environmental conditions, as far as known, 

 showed no divergence of interest. 



In the season of 1906, the first census showed an increase of the number and 

 strength of the isolated groups and changes in the distribution of the population 



96 — S«y5KiS|» — 1>» 106 



1099 Males. Total 2300. 1201 Females. 



Fig. 115. — First census at Ticrra Blanca in 1905, showing restriction of the 

 population to relatively narrow conditions centering around biotype 7. 



.22 



-77 

 -192 

 -311 

 ■196 



Fig. 116. — Second census at Tlerra Blanca in 1905, showing somewhat increased 

 array and the existence in the females of two highly divergent isolated groups. 



as a whole, especially in the males. In this sex, biotypes 4 and 5 were strongly 

 developed ; 6a was present as a marked group isolated from the population, and 

 14 and 15 were also isolated and of considerable strength in the population. 

 The females showed almost complete absence of biotypes, with the exception of 7, 

 and an isolated group composed of the extremes of 4 and some few individuals 

 of 9, both standing well apart from the mass of the population. In the second 

 determination of the condition in the population a sharp change had taken place 

 in that the mass of the population in range and in the groups that were present 

 had been much extended in both sexes and was about equal in the array of 

 biotypes presented. In both, 5, 6a, and 8 were strongly present; 4 was strong in 

 the males but weak in the females ; t strong in both ; and in the males, 14 was 

 present as a well-marked isolated group. The conditions found are given in 

 figures 117 and 118. The only item to be noted in the environmental conditions 



