280 



The Mechanism of Evolution" in Leptinotaksa 



In the second generation of this year the distribution of the population over 

 the range of values was much different from that of the first. The line com- 

 posed of biotypes 1, 2, 3, and 4 was nearly restored in the population, completely 

 so in the males and with only the lack of 2 and 3 in the females, while in both 

 sexes 10 was present and isolated, as was also 11, while 5, 6, and 8 were wanting 

 or weak in their manifestation. The conditions at this location seemed in this 

 year to be much more variable than at the Chapultepec colony, and this seemed 

 in some respects to be reflected in the behavior of the population. The findings 

 in the censuses are shown in figures 68 and 69. 



/ \\ 



4 8 12 



737 Females. 



V^^—^l 



Males. Total 1390. 



Fig. 68. — Census of first annual generation at Texcoco In 1904, showing condi- 

 tion of the pronotal pattern. 



738 Males. Total 1352. 614 Females. 



Fig. 69. — Census of second annual generation at Texcoco in 1904, showing con- 

 dition in the pronotal pattern and increased variation thereof, but with marked 

 gaps between the extreme members. 



In the season of 1905 the population of the colony showed even more diversity 

 in the range and in the number of isolated groups than in the previous season. 

 In the first census made this year, biotype 1 was present as a conspicuously iso- 

 lated group, not numerous, but vigorous in appearance, and would easily have 

 passed in the population as a distinct species. In the males, biotypes 12, 11, 10, 

 and 9 were all present and formed a continuous series, but in the females 11 

 was wanting, leaving 10 and 12 separated in groups. Biotype 8 was practically 

 wanting in both sexes and 6 was absent from the female population, and repre- 

 sented in the males by a small isolated group. Biotype 5 was present in both, 

 but in the males it was broken, the two extremes existing, with the average con- 

 ditions wanting, while the females presented a uniform array in this series. 



