IN a former communication (NATURE, vol. ix. p. 361) I promised to advance what seems to me a probable causeâ additional to those already knownâ of the reduction of useless structures. As before stated, it was suggested to me by the penetrating theory proposed by Mr. Darwin (NATURE, vol. viii. pp. 432 and 505), to which, indeed, it is but a supplement. Epitomising Mr. Darwin's conception as to the dwarfing influence of impoverished conditions, progressively reducing the average size of a useless structure, by means of free intercrossing ; the present cause may be denned as the mere cessation of the selective influence from changed condition of life.