284 The Mechanism of Evolution in Leptinotaesa 



In the season of 1907 this favorable condition existing in the population at 

 the close of 1906, and the still more favorable conditions in the early portion of 

 1907, with plentiful, well-distributed precipitation and the higher temperatures 

 of late May and June, were the series of conditions that preceded and ensued 

 during the production of the first generation in this year, and which showed the 

 unusual condition for this location of almost no isolated groups and widely 

 ranging distribution of the character over the possible range of manifestations. 

 In both sexes biotypes 1, 2, 3, 4, 5, 7, 9, 10, 11, and 12 were present in full inten- 

 sity, while 6 and 8 were weakly present in the males and in the females as the 

 only two isolated groups in the entire popiujation. The condition of the popula- 

 tion in this generation is for once not unlike that in the Chapultepec colony in 

 corresponding generation. 



The second generation in this season showed the rapidity with which the 

 population may change its aspect on the advent of an efficient cause. In this 

 generation the solid, widely distributed condition of the pattern was again dis- 

 turbed, and in its stead there was a narrowly restricted central mass of the 

 population, and several sharply isolated groups. In the population biotypes 2, 

 4, 5, 6, and 11 were entirely wanting in both sexes; 9 was present as a weak 

 development in both sexes; 8 was strongly marked in both males and females. 

 Biotypes 1, 3, 10, and 12 showed distinctly segregated groups of considerable 

 numbers in each instance, so that the aspect of the population at this time 

 presented an entirely different one from that shown in the first generation of 

 this season. The record of the two censuses are shown in figures 74 and 75. 



The season of 1908 was favorable at this location on the whole, and from the 

 point of view of climatic conditions there should have been a condition not 

 unlike that in the Chapultepec colony at the same time. The census of the first 

 generation for this season showed the characteristic, much-restricted central 

 mass of the population and three large groups of isolated patterns. In the cen- 

 sus biotypes 1 and 2 as a large group, 12 and 10 in the males, and 12 in the 

 females were sharply separated from the rest of the population. Within the 

 mass of the population biotypes 6 and 8 showed strong development in both sexes. 

 The second generation at the time of census-taking showed essentially the same 

 state as in the first generation, with the entire absence of biotype 12, and some 

 minor changes in the rest of the population. The combined group, consisting 

 of biotypes 1 and 2, was still strong and prominent in the population, in marked 

 contrast to the condition of these groups in some other generations that had been 

 examined earlier in the series. The results of the censuses for this season I have 

 given in figures 76 and 77. 



In the following year (1909) the colony was visited once in the early portion of 

 the season, and the last census of the population made in the series at Texcoco. 

 The condition in the population then found showed as in the first generation 

 of 1907 that the conditions or something in the complex, either within or with- 

 out the organisms, had produced an array of pattern conditions that was on the 

 whole the widest that had been found in this location during the observations, 

 and on the whole it presented a uniform distribution over the range of the 

 possible arrangements. No marked or large groups isolated from the popula- 

 tion were found and the three shown in the males are unimportant and few in 

 number. The findings in this census are shown in figure 78. 



