


TOWER, DARWINISM. 429 
an elimination that may have been determinative in its direction 
due to physical forces in the environment. A number of instances 
of this general type of action have come to my attention especially 
while living and working in the tropics and in the deserts. A 
few of them have been of such a nature and under conditions 
where it was profitable to continue the observation. All of them 
gave results that were identical in principle and one example must 
suffice for the purposes of this brief paper. 
Leptinotarsa panamensis, inhabiting the humid areas in the 
Isthmus of Darien, Panama and northward into Costa Rica, is 
interesting from the fact that the egg-shell is thin, delicate and 
only slightly resistant to desiccating influences. The egg dries up 
easily and a relatively brief period of dryness will so desiccate 
the eggs of this insect that they do no hatch. After they“have 
hatched, the same degree of dryness, even if continued through 
the entire larval period is not fatally antagonistic to their growth 
and development, and the same is true of the adults. This parti- 
cularly susceptible period in the life cycle of these organisms is 
one that, through differences in the character and texture of the egg 
membranes, might conceivably be the basis of directive elimination 
as the result of the physical forces of the environment. It may be 
added at this point that the statements concerning the behavior 
of the egg of this species with reference to the water-balance relations 
of the environment are made from laboratory experimental tests 
and are not based upon interpretations of conditions found in 
nature. With this as the basis, it is of interest to see what happens 
in nature. 
This species is fairly abundant all over the Republic of Panama 
and is constantly found at many locations in the Canal Zone. Its 
distribution is erratic and varies from year to year and even in diffe- 
rent portions of the same season. In the northern part of Panama, the 
high Sierra Chiriqui, with their extremely humid eastern slopes 
and relatively dry western slopes, which have, nevertheless, many 
humid barrancas, afforded opportunity to test the questions of 
elimination with this species, due to the easily desiccated eggs. 
In 1912 a reconnaissance was made of the Chiriqui region and 
food plants were transplanted as well as seeds planted in twelve 
