, on the contrary, the
unity of life is to be found solely in the impetus that pushes it along
the road of time, the harmony is not in front, but behind. The unity is
derived from a _vis a tergo_: it is given at the start as an impulsion,
not placed at the end as an attraction. In communicating itself, the
impetus splits up more and more. Life, in proportion to its progress, is
scattered in manifestations which undoubtedly owe to their common origin
the fact that they are complementary to each other in certain aspects,
but which are none the less mutually incompatible and antagonistic. So
the discord between species will go on increasing. Indeed, we have as
yet only indicated the essential cause of it. We have supposed, for the
sake of simplicity, that each species received the impulsion in order to
pass it on to others, and that, in every direction in which life
evolves, the propagation is in a straight line. But, as a matter of
fact, there are species which are arrested; there are some that
retrogress. Evolution is not only a movement forward; in many cases we
observe a marking-time, and still more often a deviation or turning
back. It must be so, as we shall show further on, and the same causes
that divide the evolution movement often cause life to be diverted from
itself, hypnotized by the form it has just brought forth. Thence results
an increasing disorder. No doubt there is progress, if progress mean a
continual advance in the general direction determined by a first
impulsion; but this progress is accomplished only on the two or three
great lines of evolution on which forms ever more and more complex, ever
more and more high, appear; between these lines run a crowd of minor
paths in which, on the contrary, deviations, arrests, and set-backs, are
multiplied. The philosopher, who begins by laying down as a principle
that each detail is connected with some general plan of the whole, goes
from one disappointment to another as soon as he comes to examine the
facts; and, as he had put everything in the same rank, he finds that, as
the result of not allowing for accident, he must regard everything as
accidental. For accident, then, an allowance must first be made, and a
very liberal allowance. We must recognize that all is not coherent in
nature. By so doing, we shall be led to ascertain the centres around
which the incoherence crystallizes. This crystallization itself will
clarify the rest; the main directions will appea