ch might have stopped
the soaring course of animal life. There is one peculiarity with which
we cannot help being struck when glancing over the fauna of primitive
times, namely, the imprisonment of the animal in a more or less solid
sheath, which must have obstructed and often even paralyzed its
movements. The molluscs of that time had a shell more universally than
those of to-day. The arthropods in general were provided with a
carapace; most of them were crustaceans. The more ancient fishes had a
bony sheath of extreme hardness.[59] The explanation of this general
fact should be sought, we believe, in a tendency of soft organisms to
defend themselves against one another by making themselves, as far as
possible, undevourable. Each species, in the act by which it comes into
being, trends towards that which is most expedient. Just as among
primitive organisms there were some that turned towards animal life by
refusing to manufacture organic out of inorganic material and taking
organic substances ready made from organisms that had turned toward the
vegetative life, so, among the animal species themselves, many contrived
to live at the expense of other animals. For an organism that is animal,
that is to say mobile, can avail itself of its mobility to go in search
of defenseless animals, and feed on them quite as well as on vegetables.
So, the more species became mobile, the more they became voracious and
dangerous to one another. Hence a sudden arrest of the entire animal
world in its progress towards higher and higher mobility; for the hard
and calcareous skin of the echinoderm, the shell of the mollusc, the
carapace of the crustacean and the ganoid breast-plate of the ancient
fishes probably all originated in a common effort of the animal species
to protect themselves against hostile species. But this breast-plate,
behind which the animal took shelter, constrained it in its movements
and sometimes fixed it in one place. If the vegetable renounced
consciousness in wrapping itself in a cellulose membrane, the animal
that shut itself up in a citadel or in armor condemned itself to a
partial slumber. In this torpor the echinoderms and even the molluscs
live to-day. Probably arthropods and vertebrates were threatened with it
too. They escaped, however, and to this fortunate circumstance is due
the expansion of the highest forms of life.

In two directions, in fact, we see the impulse of life to movement
getting the upper hand