and they are an easy way of
getting out of the difficulty; but unfortunately, they are not supported
by evidence. Local catastrophes have undoubtedly occurred, as
earthquakes and volcanic eruptions, of which the effects can be
sufficiently seen; but why suppose any universal catastrophe, when the
ordinary progress of nature suffices to account for the phenomena?
Nature is never _brusque_. She proceeds slowly step by step,
and this with occasional local catastrophes will remove all our
difficulties."[245]

In his fourth chapter Lamarck points out that animals move themselves,
or parts of themselves, not through impulsion or movement communicated
to them as from one billiard ball to another, but by reason of a cause
which excites their irritability, which cause is within some animals and
forms part of them, while it is wholly outside of others.[246]

I should again warn the reader to be on his guard against the opinion
that any animals can be said to live if they have no "inward motion" of
their own which prompts them to act. We cannot call anything alive which
moves only as wind and water may make it move, but without any impulse
from within to execute the smallest action and without any capacity of
feeling. Such a creature does not look sufficiently like the other
things which we call alive; it should be first shown to us, so that we
may make up our minds whether the facts concerning it have been truly
stated, and if so, what it most resembles; we may then classify it
accordingly.

"Some animals change their place by creeping, some by walking, some by
running or leaping; others again fly, while others live in the water and
swim.

"The origin of these different kinds of locomotion is to be found in the
two great wants of animal life: 1, the means of procuring food; 2, the
search after mates with a view to reproduction.

"Since then the power of locomotion was a matter affecting their
individual self-preservation, as well as that of their race, the
existence of the want led to the means of its being gratified."[247]

Lamarck is practically at one with Dr. Erasmus Darwin, that modification
will commonly travel along three main lines which spring from the need
of reproduction, of procuring food, and (Dr. Darwin has added) the power
of self-protection; but Dr. Darwin's treatment of this part of his
subject is more lucid and satisfactory than Lamarck's, inasmuch as he
immediately brings forward instances of various mo