The lateness and irregularity of the wisdom teeth are sometimes supposed to indicate their gradual disappearance through want of room in a diminishing jaw. But a note on Tasmanian skulls in the _Catalogue of the College of Surgeons_ (p. 199) shows that this lateness and irregularity have been common among Tasmanians as well as among civilized races, so that the change can hardly be attributed to the effects of disuse under civilization. BLIND CAVE-CRABS. The cave-crabs which have lost their disused eyes but _not the disused eye-stalks_ appear to illustrate the effects of natural selection rather than of disuse. The loss of the exposed, sensitive, and worse-than-useless eye, would be a decided gain, while the disused eye-stalk, being no particular detriment to the crab, would be but slightly affected by natural selection, though open to the cumulative effects of disuse. The disused but better protected eyes of the blind cave-rat are still "of large size" (_Origin of Species_, p. 110). NO CONCOMITANT VARIATION FROM CONCOMITANT DISUSE. It is but fair to add that these instances of the cave-crab's eye-stalk and the closely-packed teeth are put forward by Mr. Spencer with the more immediate object of proving that there is "no concomitant variation in co-operative parts," even when "formed out of the same tissue, like the crab's eye and its peduncle" (pp. 12-14, 23, 33). It escapes his notice, however, that in two out of his three cases it is _disuse_, or _diminished use_, which fails to cause concomitant variation or proportionate variation. THE GIRAFFE, AND NECESSITY FOR CONCOMITANT VARIATION.