CASTLE. — INBREEDING, CROSS-BREEDING, AND SELECTION. 733 



inbred continuously. This indeed seemed to be the result of our earlier 

 experiments, but more extended observations indicate that close-breeding 

 is not necessarily attended by decreased fertility, in this animal, but that 

 particular degrees of fertility are transmitted in certain families irrespec- 

 tive of the consanguinity of the parents. 



Material and Methods. 



The organism used in this experiment in close-breeding was Drosophila 

 ampelophila Low, a small dipterous insect known under various popular 

 names such as "the little fruit fly, pomace fly, vinegar fly, wine fly, and 

 pickled fruit fly." These appellations indicate the class of substances in 

 which the insect breeds. During the autumn it will commonly be found 

 in the vicinity of fermenting fruit and decaying vegetable material. It is 

 especially attracted by over-ripe grapes in the process of vinous fermen- 

 tation. Through breaks in the skin of such grapes it lays its eggs, the 

 resulting larvae feeding on the pulp. 



A brief sketch of the life history of Drosophila will show that it is a 

 form well adapted for experiments on inbreeding. The life cycle is 

 rapidly completed, making it easy to obtain a large number of generations 

 in a comparatively short period. The flies can be kept breeding the year 

 round at ordinary room temperature on various fruits, some of which can 

 be readily obtained at any season. 



The imagoes become sexually mature within thirty-nine hours after 

 hatching. The female can lay eggs in about forty-eight hours after 

 emergence. The time may be, however, somewhat longer, depending 

 probably on the attractiveness of the material furnishing a place for 

 oviposition. Fermenting and decaying fruit seem to stimulate the laying 

 of the eggs. At the end of three or four days the larvae appear and 

 eat voraciously. The duration of the larval state varies considerably, 

 but under favorable circumstances it may be completed within three days 

 and the pupal state assumed. The larvae commonly seek dry places 

 for pupation, usually leaving the food substance and crawling up the 

 side of the vessel in which they are contained. The duration of the 

 pupal condition, again, varies considerably, but may be completed at the 

 end of about three days. 



Allowing thus for the egg, larval, and pupal stages about three days 

 each, and two for the imago to lay eggs, it follows that the whole life 

 cycle may be completed within eleven or twelve days. But the average 

 length of this cycle, when the whole year with its varying conditions is 



