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.