Ajstalysis of Hbteeogenbitt in the Population 393 



with the methods of the census-taker. Both locations present differing climatib 

 complexes, different topographic conditions, and, as might be expected, pre- 

 sented different aspects in the character of the population. A priori, it might 

 be expected that the populations would differ in the two locations. The problem 

 is, how and why do they differ? The "how different" is of no importance, 

 excepting as it helps in the solution of the method of becoming different and the 

 analysis of this operation. 



THE CHALCICOMULA COLONY. 



This location on the western slope of the volcano of Citlaltepetl differs in 

 many respects from the location in the valley of Mexico and the colony at 

 Puebla. Situated in the lee of the huge volcanic pile, at an altitude of 8,500 

 feet, it was, as a result of the physiographic conditions, a dry, cold, grass-covered 

 plain, with a short growing-season and cold nights, even in the best of seasons. 

 In this general region the development of the colonies of this form were not at 

 any time on a par with those at Puebla or at Chapultepec in the valley of 

 Mexico. The location chosen for examination was 3 miles west and north of the 

 town of Chalcicomula and about 1 mile west of the Barrio de San Augustin, in 

 the open plain, with rainy-season pools. Observations were begun in 1904 and 

 continued until the middle of 1910, covering 13 generations in the location. 

 The censuses at this location were not as satisfactory as those at other locations, 

 owing to the small numbers and the frequent difficulty of being sure that a con- 

 siderable number of the previous population had not persisted and formed a 

 part of those present at the second census in the season. This was due to the 

 frequent spell of low temperature in the summer months, which kept the indi- 

 viduals alive, inactive, and present in the population longer than in other loca- 

 tions. This error exists in any census determinations and can hardly be com- 

 pensated for or eliminated unless the individual by being killed is put beyond 

 the possibility of again entering into the count. The determinations, defective 

 as they may be, show decided differences between this colony and those previously 

 studied. 



In the first year of observation (1904), the first census showed a small popu- 

 lation, which was centered around biotype 7 in both sexes and with 8 well devel- 

 oped in both. TSo divergent or isolated groups were found in the popula- 

 tion. The records are shown in figures 88 and 89. The second census, made 

 in September, showed the same general condition, a small population, aggregated 

 closely about biotype 7, and in this generation 8 was not represented any more 

 than the others that center immediately about the pivotal biotype. Comparison 

 of these two censuses with those made in the same year at the other locations 

 shows a quite different condition than exists elsewhere. 



In the season of 1905 the first census made showed in the males a strong 

 development of biotype 8 and something of 4 and 5, but in the female the condi- 

 tion was not changed. No isolated groups of any kind were found in the loca- 

 tion. The second determination for the year showed in both sexes about the 

 same condition that existed in the males of the previous generation, this time 

 without divergent or isolated groups. The records of the conditions found are 

 shown in figures 90 and 91. The two generations of this year compare well in 

 range and numbers with those of the previous years, the four showing that the 

 location is more constant in the appearance of the population than any of the 

 other locations examined. 



