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.