DISUSE AND DEGENERATION. 39 



receive and act upon such a stimulus. In certain insects the germ-cells 

 are all practically matured before or by the time the wings are expanded 

 and ready for use. This, however, is not the case with Drosophila. 

 Not only are the germ-cells not all matured by the time it becomes adult, 

 but they are in all stages of development and continue to mature, a few 

 at a time, for a month thereafter. In these experiments I rarely used 

 as parents the individuals coming from first-laid eggs, so that there were 

 strong chances of my using affected germ-plasm if such exists. Any 

 experiment, such as this, is always open to the criticism that it has not 

 been sufficiently long continued, but I am sure that most will agree 

 that 43 generations, combined with microscopic measurements and the 

 delicacy of biometric analysis, ought to give a satisfactory indication of 

 what is taking place. 



The second set of considerations might explain any observed degen- 

 erations without reference to the disuse. Excessive inbreeding was 

 practiced, sister usually being bred to brother. This was necessary for, 

 if I had planned to stop at this point and had wished to entirely avoid 

 inbreeding, I would have needed more than 8 trillion flies with which to 

 start the work. Inbreeding is supposed to lead to degeneration and might 

 thus be solely accountable for degeneration, or it might assist disuse. 

 Unnatural conditions might have adversely affected thefflies. Confine- 

 ment itself, apart from the entailed disuse, might at least help to bring 

 about degeneration. Furthermore, I kept the insects breeding winter 

 and summer, with no rest for hibernation and with no change of food. 

 There was no conscious selection favoring perfect and large wings, as 

 all measurements of this strain were made quite recently and the vari- 

 ations in wing-dimensions are not readily appreciable, hence the removal 

 of selection in favor of good wings might result in panmixia and conse- 

 quent degeneration. Finally, I was constantly on the lookout for signs 

 of degeneration, as I hoped and still do hope to produce a wingless Dro- 

 sophila. My desire might have influenced my actions and an unconscious 

 selection on my part might have reduced the size of the wings without 

 disuse playing a part. 



The only necessary answer to this second set of considerations is that, 

 in spite of the possibility of the degenerating effect of disuse being helped 

 by inbreeding, unnatural conditions, panmixia, or selection, there has 

 been no degeneration. 



Evidence of degeneration was sought for by carefully measuring the 

 expanded wings of the individuals belonging to successive stages of the 

 experiment. In making these measurements one may not mix the sexes 

 because of the sexual difference in size. Therefore the females alone 

 were used, since among insects it is more commonly the females which 

 have degenerate wings. The results are shown in table 37, where 33 

 units of length equal 1 mm. 



