adations, sometimes in one character and sometimes in another, but all
leading towards our {222} present perfect type. By small and doubtful steps
such as these, nature, as we may confidently believe, has progressed on her
grand march of improvement and development.

A similar line of reasoning is as applicable to separate organs as to the
whole organisation. A writer[532] has recently maintained that "it is
probably no exaggeration to suppose that, in order to improve such an organ
as the eye at all, it must be improved in ten different ways at once. And
the improbability of any complex organ being produced and brought to
perfection in any such way is an improbability of the same kind and degree
as that of producing a poem or a mathematical demonstration by throwing
letters at random on a table." If the eye were abruptly and greatly
modified, no doubt many parts would have to be simultaneously altered, in
order that the organ should remain serviceable.

But is this the case with smaller changes? There are persons who can see
distinctly only in a dull light, and this condition depends, I believe, on
the abnormal sensitiveness of the retina, and is known to be inherited.
Now, if a bird, for instance, received some great advantage from seeing
well in the twilight, all the individuals with the most sensitive retina
would succeed best and be the most likely to survive; and why should not
all those which happened to have the eye itself a little larger, or the
pupil capable of greater dilatation, be likewise preserved, whether or not
these modifications were strictly simultaneous? These individuals would
subsequently intercross and blend their respective advantages. By such
slight successive changes, the eye of a diurnal bird would be brought into
the condition of that of an owl, which has often been advanced as an
excellent instance of adaptation. Short-sight, which is often inherited,
permits a person to see distinctly a minute object at so near a distance
that it would be indistinct to ordinary eyes; and here we have a capacity
which might be serviceable under certain conditions, abruptly gained. The
Fuegians on board the {223} Beagle could certainly see distant objects more
distinctly than our sailors with all their long practice; I do not know
whether this depends on nervous sensitiveness or on the power of adjustment
in the focus; but this capacity for distant vision might, it is probable,
be slightly augmented by succes