278 The Mechanism of Evolution in Leptinotaesa 



The second census was taken at the optinram of the season in the second 

 annual generation, when the conditions were somewhat more severe than they 

 had been in this portion of the season in previous years. In this census there 

 were no isolated groups in either sex; biotypes 10, 11 and 13 were wanting in 

 the population in both sexes; biotype 8 was reduced to the merest trace, and 

 might have been absent entirely; while biotypes 5 and 6 were variable in their 

 presence in the sexes, but were certainly present in both. The general condi- 

 tion in the population in this year shows a uniform restriction in the range 

 which might in some measure be connected up with the prevailingly harder con- 

 ditions of the environment in the location during this season. The findings in 

 the distribution of the population are given in figu,res 64 and 65. 



These 13 censuses, taken in the same location, when each of the two annual 

 generations were at their maximum, do not show for the location uniformity in 

 the population as a mass, but diversity. During the progress of the observations 

 the climatic conditions were given as full study and measurement as possible, 

 and while it would have been vastly better to have had measurements made on 

 the spot, by methods better suited to the purposes of the study, the government 

 observations are the best on the whole that are available for the entire period 

 of the observations, and do show in rough fashion the march of events in the 

 habitat. 



The population is constantly shifting in this location, and has shown in the 

 distribution of the quality chosen for examination — conditions which if seen but 

 once would have been quite differently interpreted. It would in this instance 

 not be difficult to build up from the data of this set of observations rather plausi- 

 ble arguments concerning the action of the environmental conditions in the pro- 

 duction of the diversified arrays presented in the different censuses. I have 

 never been enthusiastic in this, because all that I have been able to perceive in 

 the situation is that the determination in nature has given me data concerning 

 the sequence of events in two series of phenomena that were passing at the same 

 place and time, and in some instances the oscillations in the one coincided in 

 some respects with the oscillations in the other, but in so far as I am able to dis- 

 cover, there exists in these determinations plausibilities and suggestive situa- 

 tions that may well be the motive for exact experimental analysis in nature and in 

 the laboratory. It is situations of this sort that have given the basis for numer- 

 ous arguments in faunistic and ecological literature, as well as in the field of 

 evolution essayism, in which the argument is from effect to cause. 



THE TEXCOCO COLONY. 



The records at this colony cover the period from 1903 to the middle of 1909, 

 13 censuses in the population being made, of which the first two were from col- 

 lections made in the summer of 1903. The location and its general topographic 

 setting have been previously described and differ from the Chapultepec location 

 in many respects. For a record of the climatic conditions prevailing at this 

 location I have been indebted to the local observers connected with the service 

 in the state of Mexico. These are not so complete as those from the observatory 

 at Tacubaya, and in the main are monthly averages, with distribution of some of 

 the elements. These records, while more might be desired, are far better than 

 none at all and are the best that one can do in work of this sort where trained 

 observers are not available to make the proper measurements. 



