necessarily entail _some one specific set
of alterations in the other parts_. Cuvier says, "None of these parts
can be changed without affecting the others; and consequently, each
taken separately, indicates and gives all the rest." The first of these
propositions may pass, but the second, which it is alleged follows from
it, is not true; for it implies that "all the rest" can be severally
affected in only one way and degree, whereas they can be affected in
many ways and degrees. To show this, we must again have recourse to a
mechanical analogy.

If you set a brick on end and thrust it over, you can predict with
certainty in what direction it will fall, and what attitude it will
assume. If, again setting it up, you put another on the top of it, you
can no longer foresee with accuracy the results of an overthrow; and on
repeating the experiment, no matter how much care is taken to place the
bricks in the same positions, and to apply the same degree of force in
the same direction, the effects will on no two occasions be exactly
alike. And in proportion as the aggregation is complicated by the
addition of new and unlike parts, will the results of any disturbance
become more varied and incalculable. The like truth is curiously
illustrated by locomotive engines. It is a fact familiar to mechanical
engineers and engine-drivers, that out of a number of engines built as
accurately as possible to the same pattern, no two will act in just the
same manner. Each will have its peculiarities. The play of actions and
reactions will so far differ, that under like conditions each will
behave in a somewhat different way; and every driver has to learn the
idiosyncrasies of his own engine before he can work it to the greatest
advantage. In organisms themselves this indefiniteness of mechanical
reaction is clearly traceable. Two boys throwing stones will always
differ more or less in their attitudes, as will two billiard-players.
The familiar fact that each individual has a characteristic gait,
illustrates the point still better. The rhythmical motion of the leg is
simple, and on the Cuvierian hypothesis, should react on the body in
some uniform way. But in consequence of those slight differences of
structure which consist with identity of species, no two individuals
make exactly similar movements either of the trunk or the arms. There
is always a peculiarity recognizable by their friends.

When we pass to disturbing forces of a non-mechan