wards
mediocrity; while even if it were somewhat greater, we can see many
possible contributory causes to its production. In the case of civilised
man's diminished jaw, there may well be some correlation between the jaw
and the brain, seeing that increased mental activity would lead to the
withdrawal of blood and of nervous energy from adjacent parts, and might
thus lead to diminished growth of those parts in the individual. And in
the case of pet-dogs, the selection of small or short-headed individuals
would imply the unconscious selection of those with less massive
temporal muscles, and thus lead to the concomitant reduction of those
muscles. The amount of reduction observed by Darwin in the wing-bones of
domestic ducks and poultry, and in the hind legs of tame rabbits, is
very small, and is certainly no greater than the above causes will well
account for; while so many of the external characters of all our
domestic animals have been subject to long-continued artificial
selection, and we are so ignorant of the possible correlations of
different parts, that the phenomena presented by them seem sufficiently
explained without recurrence to the assumption that any changes in the
individual, due to disuse, are inherited by the offspring.

_Supposed Effects of Disuse among Wild Animals._

It may be urged, however, that among wild animals we have many undoubted
results of disuse much more pronounced than those among domestic kinds,
results which cannot be explained by the causes already adduced. Such
are the reduced size of the wings of many birds on oceanic islands; the
abortion of the eyes in many cave animals, and in some which live
underground; and the loss of the hind limbs in whales and in some
lizards. These cases differ greatly in the amount of the reduction of
parts which has taken place, and may be due to different causes. It is
remarkable that in some of the birds of oceanic islands the reduction is
little if at all greater than in domestic birds, as in the water-hen of
Tristan d'Acunha. Now if the reduction of wing were due to the
hereditary effects of disuse, we should expect a very much greater
effect in a bird inhabiting an oceanic island than in a domestic bird,
where the disuse has been in action for an indefinitely shorter period.
In the case of many other birds, however--as some of the New Zealand
rails and the extinct dodo of Mauritius--the wings have been reduced to
a much more rudimentary condition,