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 

 antennae. 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- 



