itself so as to take the greatest possible
advantage of its immediate environment with the least possible trouble.
Accordingly, the act by which life goes forward to the creation of a new
form, and the act by which this form is shaped, are two different and
often antagonistic movements. The first is continuous with the second,
but cannot continue in it without being drawn aside from its direction,
as would happen to a man leaping, if, in order to clear the obstacle, he
had to turn his eyes from it and look at himself all the while.

Living forms are, by their very definition, forms that are able to live.
In whatever way the adaptation of the organism to its circumstances is
explained, it has necessarily been sufficient, since the species has
subsisted. In this sense, each of the successive species that
paleontology and zoology describes was a _success_ carried off by life.
But we get a very different impression when we refer each species to the
movement that has left it behind on its way, instead of to the
conditions into which it has been set. Often this movement has turned
aside; very often, too, it has stopped short; what was to have been a
thoroughfare has become a terminus. From this new point of view, failure
seems the rule, success exceptional and always imperfect. We shall see
that, of the four main directions along which animal life bent its
course, two have led to blind alleys, and, in the other two, the effort
has generally been out of proportion to the result.

Documents are lacking to reconstruct this history in detail, but we can
make out its main lines. We have already said that animals and
vegetables must have separated soon from their common stock, the
vegetable falling asleep in immobility, the animal, on the contrary,
becoming more and more awake and marching on to the conquest of a
nervous system. Probably the effort of the animal kingdom resulted in
creating organisms still very simple, but endowed with a certain freedom
of action, and, above all, with a shape so undecided that it could lend
itself to any future determination. These animals may have resembled
some of our worms, but with this difference, however, that the worms
living to-day, to which they could be compared, are but the empty and
fixed examples of infinitely plastic forms, pregnant with an unlimited
future, the common stock of the echinoderms, molluscs, arthropods, and
vertebrates.

One danger lay in wait for them, one obstacle whi