252 WILLIAM LAWRENCE TOWER. 



capable thereof, and we are determining in the experiment only 

 some aspects of the composition of the population of the species 

 in its original habitat. 



2. That the introduced populations are uniform in composi- 

 tion, but that as the result of the altered conditions of life, 

 variations are induced, of which only those whose reactions 

 enable them to resist desiccation have survived ; a natural selec- 

 tion of the fit by the physical environic factors. 



3. That the population has latent within it the factor for water 

 retention, and that this latency from an ancestral conditions 

 under the conditions of the new habitat, is reactivated, mutates 

 to the ancestral state in the materials of the experiments. 



4. That the water relation in connection with hibernation is 

 one due to some physico-chemical relation in the living colloids, 

 that exists in one of two aspects, a condition of the substance, 

 its type of manifestation depending upon the nature of the 

 conditions of the medium. 



There may be other possible methods whereby the condition 

 is produced in the experiments, but these seem to me the most 

 obvious possibilities. 



That the results are due to the isolation under experiment 

 from the introduced population of a pure line does not seem 

 probable, or at all possible under the conditions of these experi- 

 ments. All the basal populations introduced came from cultures 

 in which the reproductive cycle was known to be homozygous 

 with respect to the rhythm of the two generations, and since any 

 heterozygousness in this rhythm is at once manifested and very 

 obvious in the cultures, and was also repeatedly tested for in 

 the stock cultures, and none found, the evidence is against the 

 assumption. Further all the introduced stocks were taken from 

 the original stocks on emergence from hibernation, after all the 

 elimination incidental to hibernation had taken place. Were 

 there lines in the original materials with the conditions that later 

 developed in the Tucson cultures, which were isolated by the 

 selective action of the Tucson experiments, and could not later 

 survive the Chicago conditions when returned thereto; why, if 

 present in the original stocks, were they not also eliminated by 

 the conditions of the winter hibernation in the parental habitat? 



