240 The Mechanism of Evolution in Leptinotaesa 



slopes of el volcan de citlaltepetl. 



On the cold, dry, grassy slopes on the western side of this huge volcanic cone a 

 small colony was found near the station of San Andres Chalcicomula, State of 

 Puehla, Mexico. This location, although without meteorological records, is of 

 interest because of the cold, dry, tundra-like area in which it was located, the 

 short growing-season, and the generally rigorous conditions of life. Observa- 

 tions were begun in 1904 and continued until 1909, 13 generations being 

 observed. 



Observations were made at a number of other points : At Ometusco, Federal 

 District, Mexico, 1903-1905; Apizaco, Tlaxcala, Mexico, 1903-1907; Bsperanza, 

 State of Puebla, Mexico, 1904-1907; all on the open high plateau; at San 

 Martin, State of Puebla, Mexico, 1904-1906; and Tlaxcala, State of Tlax- 

 cala, Mexico, 1904-1906. The purpose of these was to discover if there were 

 different processes at work in locations other than the five chief points of obser- 

 vation. None was found, and it was not thought wise to expend time and 

 energy upon more than the five locations. At each of these colonies exactly the 

 same method of observation was applied, and as far as possible observations were 

 made at all of the colonies, especially the five important ones, at as near the 

 same time as possible. This was made fairly easy by the rather good railroads 

 operating ia that portion of the country, and without these the project would 

 have been impracticable, if not impossible. 



METHODS OF OBSERVATIONS. 



It has been the universal custom in studies of this character to make collections 

 in the locations chosen and preserve them for study later. In this way a consid- 

 erable proportion of the population of the colony is removed and an unknown 

 population of undetermined character left to continue the species at that point. 

 In organisms as localized as these are, and often inhibited from extensive dis- 

 semination by ujif riendly environs, it is conceivable and highly probable that the 

 removal of large numbers from a restricted area would introduce complications 

 not easily determined or compensated for. It was decided at the begiuning to 

 eliminate any error of this nature by not removing any of the population, or at 

 least in any numbers, and only those needed for experimental testing. This plan 

 has been rigorously followed and made necessary the determination of the condi- 

 tion of the population in the field. This was easily accomplished. Two, three, or 

 more natives were instructed to collect all of the kind of beetles they could find 

 in a given area, and those alive and uninjured were recorded by me, and the sex 

 and type of variations presented by the part were examined. These after exami- 

 nation and recording were kept caged until the census of the colony was complete, 

 and all were then returned to the colony to breed and do whatever other things 

 they might. I am therefore certain that the conditions described are those 

 resulting from causes proper and natural to the location, and that no error has 

 entered into the observations by elimination on my part from making collections. 



At Irolo, State of Hidalgo, Mexico, I made a test in 1904, 1905, and 1906, 

 removing a large part of the population from the colony, and by the end of 1906 

 it had become very weak in numbers and its character otherwise much altered. 

 It seemed on the whole best not to remove from any colony under observation 

 many individuals in any given generation. 



