ristics of the changes in
species that are likely to be effected, and how far they agree with what
we observe in nature.

_Divergence of Character._

In species which have a wide range the struggle for existence will often
cause some individuals or groups of individuals to adopt new habits in
order to seize upon vacant places in nature where the struggle is less
severe. Some, living among extensive marshes, may adopt a more aquatic
mode of life; others, living where forests abound, may become more
arboreal. In either case we cannot doubt that the changes of structure
needed to adapt them to their new habits would soon be brought about,
because we know that variations in all the external organs and all their
separate parts are very abundant and are also considerable in amount.
That such divergence of character has actually occurred we have some
direct evidence. Mr. Darwin informs us that in the Catskill Mountains in
the United States there are two varieties of wolves, one with a light
greyhound-like form which pursues deer, the other more bulky with
shorter legs, which more frequently attacks sheep.[37] Another good
example is that of the insects in the island of Madeira, many of which
have either lost their wings or have had them so much reduced as to be
useless for flight, while the very same species on the continent of
Europe possess fully developed wings. In other cases the wingless
Madeira species are distinct from, but closely allied to, winged species
of Europe. The explanation of this change is, that Madeira, like many
oceanic islands in the temperate zone, is much exposed to sudden gales
of wind, and as most of the fertile land is on the coast, insects which
flew much would be very liable to be blown out to sea and lost. Year
after year, therefore, those individuals which had shorter wings, or
which used them least, were preserved; and thus, in time, terrestrial,
wingless, or imperfectly winged races or species have been produced.
That this is the true explanation of this singular fact is proved by
much corroborative evidence. There are some few flower-frequenting
insects in Madeira to whom wings are essential, and in these the wings
are somewhat larger than in the same species on the mainland. We thus
see that there is no general tendency to the abortion of wings in
Madeira, but that it is simply a case of adaptation to new conditions.
Those insects to whom wings were not absolutely essential escaped a
seri