283 



The Mechanism of Evolution in Leptinotaksa 



In the season of 1906 the first generation showed no especial change from the 

 conditions shown at the close of the previous year. In both sexes biotype 5 was 

 much reduced, 6 was also reduced, and 8 represented by a few at the upper end 

 in the males and by a distinctly isolated group in the females. Biotype 1 was 

 present in both in small numbers, and 11 was found only in the females. The 

 two distributions are essentially the same. 





Fig. 72. — Census of first annual generation at Texcoco In 1906, showing con- 

 tinuation of the same condition present lu the last generation of previous year. 



. «Mi,n 



6 *^' 



17- _ ^ 



12 





33 



51 



46 



103 



42 



r 81 



*»■ I 36 



573 Males. Total 1113. 



12 

 540 Females. 



Via. 73. — Census of second annual generation at Texcoco in 1906, showing con- 

 dition with regard to pronotal pattern. 



In the census of the second generation, biotype 1 was absent in both sexes and 

 5 was represented by a mere trace ; 6 and 8 were fairly shown in the males, but 

 in the females were weakly represented, while 9, 10, and 12 were best developed 

 in the females, where 12 was found as a distinctly isolated group, the only one 

 in the entire population. The records of the two generations are shown in the 

 figures 72 and 73. Comparison of these two generations in the season of 1906 

 at Texcoco and at Chapultepec shows differences that seem permanent to the 

 two locations in the population living there. In this year, at the Chapultepec 



