Influence of Selection 207 
induce variability. It is interesting, therefore, to find that 
variability may be as marked in non-sexual forms produced 
parthenogenetically as in sexually produced forms. 
Warren has studied the variation of one of the aphids, Hyalop- 
terus trirhodus, and of Daphnia. The aphids, to consider only 
this case, were inclosed in small bags on the leaves of their na- 
tive plant, and the parents and their full-grown progeny were 
later killed and measured, the distance between the eyes and 
the antenna length being the two measurements taken. It was 
found that the external conditions, including also the natural 
changes in the food plant, produced very decided effects, espe- 
cially in size, so that it was not possible to rear successive genera- 
tions under identical conditions. Of 522 offspring registered, 
455 grew up. The death rate for the second generation was 12.8. 
The larger mothers tended to produce healthier offspring. The 
variability of the second generation was found to be greater than 
that of the parents, and this is the rule also for sexually produced 
offspring — in man, for example. In the third generation the 
variability was diminished, attributed by Warren to the poorer 
external conditions, which, affecting the size, reduce the dis- 
tance between the eyes more than they reduce the length of the 
antenne. In general, the results show that the variability of a 
parthenogenetic race is not smaller than that of sexually pro- 
duced forms. Casteel and Phillips, have found in the hive bee 
that the males or drones that develop from unfertilized eggs are 
more variable than the females that come from the fertilized 
eggs. 
The Results of Selection and Hybridization of Wild Elementary 
Species 
Domesticated animals and plants in different countries often 
show differences other than those that can be attributed to differ- 
ences in climate, etc. It has been stated that in some cases the 
domesticated forms bear a close resemblance to the local wild 
races of the country, or else show traces of such a similarity. 
The results may be due to man having brought under domestica- 
