_all_ those many components of the hind-quarters which severally require
re-adjustment. It is useless to reply that an increment of length in the
fore-legs or neck might be retained and transmitted to posterity,
waiting an appropriate variation in a particular bone or muscle in the
hind-quarters, which, being made, would allow of a further increment.
For besides the fact that until this secondary variation occurred the
primary variation would be a disadvantage often fatal; and besides the
fact that before such an appropriate secondary variation might be
expected in the course of generations to occur, the primary variation
would have died out; there is the fact that the appropriate variation of
one bone or muscle in the hind-quarters would be useless without
appropriate variations of all the rest--some in this way and some in
that--a number of appropriate variations which it is impossible to
suppose.

Nor is this all. Far more numerous appropriate variations would be
indirectly necessitated. The immense change in the ratio of
fore-quarters to hind-quarters would make requisite a corresponding
change of ratio in the appliances carrying on the nutrition of the two.
The entire vascular system, arterial and veinous, would have to undergo
successive unbuildings and rebuildings to make its channels everywhere
adequate to the local requirements; since any want of adjustment in the
blood-supply in this or that set of muscles, would entail incapacity,
failure of speed, and loss of life. Moreover the nerves supplying the
various sets of muscles would have to be proportionately changed; as
well as the central nervous tracts from which they issued. Can we
suppose that all these appropriate changes, too, would be step by step
simultaneously made by fortunate spontaneous variations, occurring along
with all the other fortunate spontaneous variations? Considering how
immense must be the number of these required changes, added to the
changes above enumerated, the chances against any adequate
re-adjustments fortuitously arising must be infinity to one.

If the effects of use and disuse of parts are inheritable, then any
change in the fore parts of the giraffe which affects the action of the
hind limbs and back, will simultaneously cause, by the greater or less
exercise of it, a re-moulding of each component in the hind limbs and
back in a way adapted to the new demands; and generation after
generation the entire structure of the hind-qu