DISUSE AND DEGENERATION. 39 
receive and act upon sucha 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 48 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 the'flies. 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. 
