244 WILLIAM LAWRENCE TOWER. 



that few of the individuals in the test survive beyond December 

 first, and none have been found alive by the end of the same 

 month. Further many of the individuals seen in these examina* 

 tions when recovered from the ground show frost crystals within 

 the tissues, and extensive disruptive actions thereof. 



Repeated tests in the different cultures at Tucson, at early 

 and late generations during their history, show that the retention 

 of water in the tissues against the influences of a strongly desiccat- 

 ing environic complex gives a series of findings that are in close 

 accord with the data derived from the survival tests of the same 

 materials at Chicago; a sharp change in this capacity between 

 introduction and the first period of hibernation, and thereafter 

 a slower increase through the following hibernation periods, 

 until the full capacity is developed, at about the sixth generation 

 after introduction. 



From all the different aspects in which this response to the 

 changed environic complex is viewed, it gives only the one 

 conclusion, that there has resulted from the sharp change in the 

 living conditions the development of a capacity to hold water 

 and not lose water in the presence of conditions in the surround- 

 ings, w^hich in the original materials causes rapid loss of water 

 with eventual death. The modification is, therefore, of a direct 

 adaptive nature, in direction and kind to meet the needs of the 

 new environment. 



What the alteration has actually been in the mechanism of 

 the organism, I do not know. Thus far I have not been able to 

 detect any anatomical modifications, or changes of a cytological 

 character that have taken place. Water loss in these animals is 

 through the dermal glandular secretions and respiratory activity. 

 As far as I can determine, there is no decrease in the dermal 

 glands that are present, either in size or number, or any altera- 

 tion in their distribution, although I have searched diligently 

 therefor. Moreover, there are no indications of a thicker cell 

 wall, either in the hypodermis or of any increased thickness in 

 the cuticular linings of the tracheal tubes. 



Whether it is due to changes in the permeability of the cell 

 membranes or to changes in the colloidal contents of the cells 

 is a matter of opinion, as I have no evidence that supports 



