The shortening of the sternum in pigeons is attributed to disuse of the flight muscles attached to it. The bone is only shortened by a third of an inch, but this represents a very remarkable reduction in proportional length, which Darwin estimates at from one-seventh to one-eighth, or over 13 per cent. This marked reduction, too, quite unlike the slight reduction of the wing-bones to which the other ends of the muscles are attached, was universal in the eleven specimens measured by Darwin; and the bone, though acknowledged to have been modified by artificial selection in some breeds, is not so open to observation as wings or legs. Even, however, if this relative shortening of the sternum remained otherwise inexplicable, it might still be as irrelevant to use and disuse as is the fact that "many breeds" of fancy pigeons have lost a rib, having only seven where the ancestral rock-pigeon has eight.[30] But the excessive reduction in the sternum is far from being inexplicable. In the first place Darwin has somewhat over-estimated it. Instead of comparing the deficiency of length with the increased length which _should_ have been acquired (since the pigeons have increased in average size) he compares it with the length of the breast-bone in the rock-pigeon.[31] By this method if a pigeon had doubled in dimensions while its breast-bone remained unaltered, the reduction would be put down as 100 per cent., whereas obviously the true reduction would be one-half, or 50 per cent. of what the bone _should be_. Avoiding this error and a minor fallacy besides, a sound estimate reduces the supposed reduction of 13 or 14 per cent. to one of 11.7 per cent., which is still of course a considerable diminution.