go a weeding-out process.
Populate the land with more animal life than it can support, or with
more vegetable forms than it can sustain, and a weeding-out process
will begin. A fuller measure of vitality, or a certain hardiness and
toughness, will enable some species to hold on longer than others,
and, maybe, keep up the fight till the struggle lessens and victory is
won.

The flame of life is easily blown out in certain forms, and is very
tenacious in others. How unequally the power to resist cold, for
instance, seems to be distributed among plants and trees, and probably
among animals! One spring an unseasonable cold snap in May (mercury
28) killed or withered about one per cent of the leaves on the lilacs,
and one tenth of one per cent of the leaves of our crab-apple tree. In
the woods around Slabsides I observed that nearly half the plants of
Solomon's-seal (_Polygonatum_) and false Solomon's-seal (_Smilacina_)
were withered. The vital power, the power to live, seems stronger in
some plants than in others of the same kind. I suppose this law holds
throughout animate nature. When a strain of any kind comes, these
weaker ones drop out. In reading the stories of Arctic explorers, I
see this process going on among their dog-teams: some have greater
power of endurance than others. A few are constantly dropping out or
falling by the wayside. With an army on a forced march the same thing
happens. In the struggle for existence the weak go to the wall. Of
course the struggle among animals is at least a toughening process. It
seems as if the old Indian legend, that the strength of the foe
overcome passes into the victor, were true. But how a new species
could arrive as the result of such struggle is past finding out.
Variation with all forms of life is more or less constant, but it is
around a given mean. Only those acquired characters are transmitted
that arise from the needs of the organism.

A vast number of changes in plants and animals are superficial and in
no way vital. It is hard to find two leaves of the same tree that will
exactly coincide in all their details; but a difference that was in
some way a decided advantage would tend to be inherited and passed
along. It is said that the rabbits in Australia have developed a
longer and stronger nail on the first toe of each front foot, which
aids them in climbing over the wire fences. The aye-aye has a
specially adapted finger for extracting insects from their
hiding-p