DISUSE AND DEGENERATION.* 



One of the several much-discussed but little-tested problems of the 

 theory of evolution is that of the inherited effects of disuse. I believe that 

 there is a pretty general idea that when a species no longer has need for 

 an organ that organ will degenerate. The explanations of this degener- 

 ation are varied, but the most popular seem to be the inheritance of 

 acquired characters, panmixia and selection. It is indisputable that in 

 the life of an individual many unused organs do degenerate, but it is far 

 from proven or even satisfactorily indicated that this ontogenetic degen- 

 eration is followed by a phylogenetic degeneration. There is no doubt 

 that many degenerate organs are not used in any way; but who can say 

 whether this disuse has preceded degeneration as a cause or merely fol- 

 lowed as a necessary consequence ? Before attempting to explain the 

 phylogenetic degeneration which follows disuse it seems desirable to find 

 a clear case of such a sequence, and this quest was the purpose of the 

 experiment with Drosophila ampelophila upon which I wish briefly to 

 report. 



These insects are normally very good fliers, possessing wings which 

 are relatively quite large. In my experiments, however, they were con- 

 fined in glass vials barely large enough to contain the food. The only 

 opportunity they had to fly was when they were transferred from one 

 vial to another. This was done only three times a week. Such flight 

 could at most not be more than 5 cm., and was, as a matter of fact, 

 rarely made, as they usually walked. 



The experiments are complicated by several facts which must be con- 

 sidered. These fall into two groups: 



First, those which might explain the absence of degeneration in the 

 wings. Disuse does not affect, during the life of an individual, the 

 wing-dimensions, for after an insect's wings are expanded there is no 

 change in them and, of course, they are not subject to the effects of use 

 and disuse before they are expanded. However, the degeneration of 

 beetle- wings when the elytra are fused, of the wings of cave insects, of 

 parasites, and of the wings of many female Lepidoptera are used as stock 

 examples of disuse. Furthermore, if there be anything in the theory 

 of hormones (of which Cunningham has recently made so much) or the 

 various forms of the memory theory of inheritance, we would expect 

 phylogenetic degeneration because of the germ-plasm receiving the news 

 that the wings are not being used, providing the plasm is in condition to 



*Paper read before the American Society of Zoologists, Baltimore meeting, 1908. 



