y
no other cause: sexual generation is perhaps only a luxury for the
plant, but to the animal it was a necessity, and the plant must have
been driven to it by the same impetus which impelled the animal thereto,
a primitive, original impetus, anterior to the separation of the two
kingdoms. The same may be said of the tendency of the vegetable towards
a growing complexity. This tendency is essential to the animal kingdom,
ever tormented by the need of more and more extended and effective
action. But the vegetable, condemned to fixity and insensibility,
exhibits the same tendency only because it received at the outset the
same impulsion. Recent experiments show that it varies at random when
the period of "mutation" arrives; whereas the animal must have evolved,
we believe, in much more definite directions. But we will not dwell
further on this original doubling of the modes of life. Let us come to
the evolution of animals, in which we are more particularly interested.

What constitutes animality, we said, is the faculty of utilizing a
releasing mechanism for the conversion of as much stored-up potential
energy as possible into "explosive" actions. In the beginning the
explosion is haphazard, and does not choose its direction. Thus the
amoeba thrusts out its pseudopodic prolongations in all directions at
once. But, as we rise in the animal scale, the form of the body itself
is observed to indicate a certain number of very definite directions
along which the energy travels. These directions are marked by so many
chains of nervous elements. Now, the nervous element has gradually
emerged from the barely differentiated mass of organized tissue. It may,
therefore, be surmised that in the nervous element, as soon as it
appears, and also in its appendages, the faculty of suddenly freeing the
gradually stored-up energy is concentrated. No doubt, every living cell
expends energy without ceasing, in order to maintain its equilibrium.
The vegetable cell, torpid from the start, is entirely absorbed in this
work of maintenance alone, as if it took for end what must at first have
been only a means. But, in the animal, all points to action, that is, to
the utilization of energy for movements from place to place. True, every
animal cell expends a good deal--often the whole--of the energy at its
disposal in keeping itself alive; but the organism as a whole tries to
attract as much energy as possible to those points where the locomotive
move