Eeactions and Products in Interspecific Crosses 



107 



tropics, essentially a montane rain-forest, while at Cuernavaca the conditions 

 are those of an upland savannah complex with intense desiccation during nine 

 months of the year, and daily desiccations of varying degrees during the rainy 

 or growing season, from June to September. Records of the evaporation-rates 

 at the two locations have been taken at different times during the progress of 

 this work as opportunity permitted, and some of these are given in table 8. 

 These are taken with the filter-paper atmometer and the free water-surface ; 

 the result recorded is the loss of water in cubic centimeters of water evaporated. 

 These and other determinations are shown in the series of curves in figure 6, 

 which show graphically the progress of the season in this respect and the rela- 

 tion of the breeding-periods to this environmental relation. It is at once clear 

 that the complexes are very different in the two locations in this respect, and the 

 rate of evaporation has been found in experiment to be one of the most important 

 agents in the environmental complex; that is, this rate of water-loss from the 



'-g 140 — T 

 "5 130 ' 

 I- '20 



Maximum atmo- 



eter record 

 Cuernavaca. 



Average daily at- 

 mometer record 

 Orizaba 



Average daily at- 



r record 

 Cuernavaca . 

 Maximum atmo- 



eter record 

 Orizaba. 



oooooooooo 



Fig. 6. — Showing differences in evaporation rates in the parental habitat of 

 li. siffnaticollis at Cuernavaca and of L. diversa at Orizaba, as determined from 

 various atmometer readings through several years. 



surface of the body, which is in general terms comparable to that shown in the 

 atmometer, alters most profoundly the action and rates of reaction in the 

 organism. It will induce changes in the trophisms of the organism most readily, 

 and reversal of the normal activities, as well as serve as an accelerator in 

 ontogeny. Thus the conditions at Cuernavaca are such as to produce rapid onto- 

 genetic development, and the species from that location have high rates of 

 reaction and short ontogenetic cycles, in signaticollis about 40 days, while at 

 Orizaba the conditions are such that there is little or no pressure of the environ- 

 ment, and as a result the ontogeny is longer in each of the two annual genera- 

 tions, and there is much overlapping and interbreeding of them. 



In the laboratory at Chicago the conditions were a close approach to those 

 found in the Orizaba location, and under these the two species come to have the 

 same rates of development. Each has, therefore, a specific rate of ontogenetic 

 progression that is characteristic of each in its original habitat, but which is 



