the principle of our demonstration. We said of
life that, from its origin, it is the continuation of one and the same
impetus, divided into divergent lines of evolution. Something has grown,
something has developed by a series of additions which have been so many
creations. This very development has brought about a dissociation of
tendencies which were unable to grow beyond a certain point without
becoming mutually incompatible. Strictly speaking, there is nothing to
prevent our imagining that the evolution of life might have taken place
in one single individual by means of a series of transformations spread
over thousands of ages. Or, instead of a single individual, any number
might be supposed, succeeding each other in a unilinear series. In both
cases evolution would have had, so to speak, one dimension only. But
evolution has actually taken place through millions of individuals, on
divergent lines, each ending at a crossing from which new paths radiate,
and so on indefinitely. If our hypothesis is justified, if the
essential causes working along these diverse roads are of psychological
nature, they must keep something in common in spite of the divergence of
their effects, as school-fellows long separated keep the same memories
of boyhood. Roads may fork or by-ways be opened along which dissociated
elements may evolve in an independent manner, but nevertheless it is in
virtue of the primitive impetus of the whole that the movement of the
parts continues. Something of the whole, therefore, must abide in the
parts; and this common element will be evident to us in some way,
perhaps by the presence of identical organs in very different organisms.
Suppose, for an instant, that the mechanistic explanation is the true
one: evolution must then have occurred through a series of accidents
added to one another, each new accident being preserved by selection if
it is advantageous to that sum of former advantageous accidents which
the present form of the living being represents. What likelihood is
there that, by two entirely different series of accidents being added
together, two entirely different evolutions will arrive at similar
results? The more two lines of evolution diverge, the less probability
is there that accidental outer influences or accidental inner variations
bring about the construction of the same apparatus upon them, especially
if there was no trace of this apparatus at the moment of divergence. But
such similarity