upposed 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.

Having unwittingly shown that lessened use of closely-connected and
co-operative parts does not cause concomitant variation in these parts,
Mr. Spencer concludes that the concomitant variation requisite for
evolution can only be caused by altered degrees of use or disuse. He
elaborately argues that the many co-ordinated modifications of parts
necessitated by each important alteration in an animal are so complex
that they cannot possibly be brought about except by the inherited
effect of the use and disuse of the various parts concerned. He holds,
for instance, that natural selection is inadequate to effect the
numerous concomitant changes necessitated by such developments as that
of the long neck of the giraffe. Darwin, however, on the contrary, holds
that natural selection alone "would have sufficed for the production of
this remarkable quadruped."[8] He is surprised at Mr. Spenc