286 



The Mechanism of Evolutiok ik Leptinotaesa 



would be increased, but the populations are rarely uniform in two separate loca- 

 tions, and this is especially so in the case of organisms that are restricted to 

 narrow habitats, and from which they do not move far. In such restricted habi- 

 tats small peculiarities may and often do develop and become the potent cause of 

 wide differences in the reaction of two supposedly like organisms to the same set 

 of conditions in the environment. There is one outcome of these determina- 

 tions of the conditions in these populations that is of interest and of some 

 moment as a practical consideration in investigations of this nature, namely, the 



\ 



4 7 14 

 510 Males. 



Total 1154 



Fio. 78. — Census of first annual generation at Texcoco In 1909, showing con- 

 dition of pronotal pattern and extension of pattern in several directions, so a 

 fairly continuous whole Is presented with only small Isolated groups in the males. 



difficulty of finding twice the same condition in the state or array of the popula- 

 tion in successive generations or seasons. The significance of this will appear 

 more evident later. 



THE TLALNEPANTLA COLONY. 



This location, in the northern portion of the valley of Mexico, the last of the 

 locations in that physiographic unit, has been described, and it presents a 

 different physiographic setting and, owing to local topography and climatic 

 conditions, the same in plan and range, but differing in intensiiy and dis- 

 tribution in the growing-season of the year. On the whole, the conditions at 

 this location are more severe than at either of the other two, and this in general 

 finds corresponding changes in the response of the population of our subject 

 living there. 



The first census recorded from that location is from a collection made in the 

 second generation of 1903 ; the remainder, to the end of 1907, have been made 

 without removal of any of the population from the location. This was especially 

 necessary in this location during most of the period of observation, owing to the 

 relatively small number of inhabitants of this particular kind found in the 

 location. 



The condition found in the second generation of the season of 1903 indicated 

 a narrowly limited range in the population without isolated groups in the 

 population. This was true for both sexes. Biotypes 1, 2, 3, 10, 11, and 1% 

 were entirely lacking in the population, and 6 and 8 were but poorly developed 



