Darwin notices that watchmakers and engravers are liable to be
short-sighted, and that short-sight and long-sight certainly tend to be
inherited.[47] But we must be careful not to beg the question at issue
by assuming that the frequent heredity of short sight necessarily covers
the heredity of artificially-produced short-sight. Elsewhere, however,
Darwin states more decisively that "there is ground for believing that
it may often originate in causes acting on the individual affected, and
may thence-forward become transmissible."[48] This impression may arise
(1) from the facts of ordinary heredity--the ancestral liability being
excited in father and son by similar artificial habits, such as reading,
and viewing objects closely as among watchmakers and engravers--or by
constitutional deterioration from indoor life, &c., acting upon a
constitutional liability of the eye to the "something like inflammation
of the coats, under which they yield" and so cause shortness of sight
by altering the spherical shape of the eye-ball. (2) Panmixia, or the
suspension of natural selection, together with altered habits, will
account for an increase of short-sight among the population generally.
(3) Long-sighted people could not work at watchmaking and engraving so
comfortably and advantageously as at other occupations, and hence would
be less likely to take to such callings.