rmediate forms could, for instance, a bat possibly have passed; but
the same question might have been asked with respect to the seal, if we
had not been familiar with the otter and other semi-aquatic carnivorous
quadrupeds. But in the case of the bat, who can say what might have been
the habits of some parent form with less developed wings, when we now
have insectivorous opossums and herbivorous squirrels fitted for merely
gliding through the air{300}. One species of bat is at present partly
aquatic in its habits{301}. Woodpeckers and tree-frogs are especially
adapted, as their names express, for climbing trees; yet we have species
of both inhabiting the open plains of La Plata, where a tree does not
exist{302}. I might argue from this circumstance that a structure
eminently fitted for climbing trees might descend from forms inhabiting
a country where a tree did not exist. Notwithstanding these and a
multitude of other well-known facts, it has been maintained by several
authors that one species, for instance of the carnivorous order, could
not pass into another, for instance into an otter, because in its
transitional state its habits would not be adapted to any proper
conditions of life; but the jaguar{303} is a thoroughly terrestrial
quadruped in its structure, yet it takes freely to the water and catches
many fish; will it be said that it is _impossible_ that the conditions
of its country might become such that the jaguar should be driven to
feed more on fish than they now do; and in that case is it impossible,
is it not probable, that any the slightest deviation in its instincts,
its form of body, in the width of its feet, and in the extension of the
skin (which already unites the base of its toes) would give such
individuals a better _chance_ of surviving and propagating young with
similar, barely perceptible (though thoroughly exercised),
deviations{304}? Who will say what could thus be effected in the course
of ten thousand generations? Who can answer the same question with
respect to instincts? If no one can, the _possibility_ (for we are not
in this chapter considering the _probability_) of simple organs or
organic beings being modified by natural selection and the effects of
external agencies into complicated ones ought not to be absolutely
rejected.

{300} <Note in original.> No one will dispute that the gliding is
most useful, probably necessary for the species in question.

{301} <Note in ori