80 CARNEGIE INSTITUTION OF WASHINGTON. 



invariably characterized by a higher osmotic concentration of its 

 fluids. They also show that on theoretical grounds higher osmotic 

 pressure of the tissue fluids is not a necessary prerequisite of successful 

 parasitism in the case of a species living under natural conditions. 



A substantial beginning has also been made by Messrs. Harris, 

 Lawrence, and Leamon in the study of the sap properties of desert 

 Loranthacese. 



GENETICS AND TAXONOMY. 

 Experimental Evolution in a Desert Habitat, by W. L. Tower. 



The rebuilding operations begun in 1915 did not, as feared, produce 

 loss in or interruption of any experiments in progress at Tucson. The 

 mortality in hibernation was the lowest it has ever been, wliile the 

 progress of the cultures during the breeding season has been highly 

 satisfactory. 



Unexpected uniformity of the physical environment in the different 

 cages and of the progress of the cultures is given by the new arrange- 

 ments, so that the entire series can now be started and come to the 

 close of the first summer generation, all at the same time, thus facihtat- 

 ing the task of checking, recording, and arranging for the next genera- 

 tion. In past years different cultures varied from one to four weeks in 

 their emergence from hibernation and in maturity, a fact which 

 greatly complicated the work. 



EXPERIMENTAL EQUIPMENT AT TUCSON. 



The rebuilding of the entire plant, begun in 1915, was continued this 

 year. A food-cage absolutely proof against contamination and of a 

 capacity large enough to supply all the food needed has been erected, 

 and as a result we had the novel experience of having a surplus of 

 food throughout the year. The grading about the entire establish- 

 ment has been completed and the top finished by a layer of 4 to 6 

 inches of fine crushed-stone screenings. Improved instrument shelters 

 of substantial construction have also been erected. These improve- 

 ments greatly reduce the cost of maintenance and also of operation. 

 The time consumed in irrigation, food production, and control has 

 been decreased to about one-fourth the expenditure in 1914, while the 

 effectiveness of the plant is greatly increased. 



INTRODUCED SPECIES OF LEPTINOTARSA. 



The largest and longest-continued culture of any pure type are those 

 of Leptinotarsa decemlineata, which came from stocks of known 

 composition and reactions hving in the mesophytic conditions at 

 Chicago. Successive introductions into the Tucson conditions from 

 the same Chicago stock all show the same progress and changes. These 



