of the two products would be natural, on the contrary,
on a hypothesis like ours: even in the latest channel there would be
something of the impulsion received at the source. _Pure mechanism,
then, would be refutable, and finality, in the special sense in which we
understand it, would be demonstrable in a certain aspect, if it could be
proved that life may manufacture the like apparatus, by unlike means, on
divergent lines of evolution; and the strength of the proof would be
proportional both to the divergency between the lines of evolution thus
chosen and to the complexity of the similar structures found in them._

It will be said that resemblance of structure is due to sameness of the
general conditions in which life has evolved, and that these permanent
outer conditions may have imposed the same direction on the forces
constructing this or that apparatus, in spite of the diversity of
transient outer influences and accidental inner changes. We are not, of
course, blind to the role which the concept of _adaptation_ plays in the
science of to-day. Biologists certainly do not all make the same use of
it. Some think the outer conditions capable of causing change in
organisms in a _direct_ manner, in a definite direction, through
physico-chemical alterations induced by them in the living substance;
such is the hypothesis of Eimer, for example. Others, more faithful to
the spirit of Darwinism, believe the influence of conditions works
_indirectly_ only, through favoring, in the struggle for life, those
representatives of a species which the chance of birth has best adapted
to the environment. In other words, some attribute a _positive_
influence to outer conditions, and say that they actually _give rise to_
variations, while the others say these conditions have only a _negative_
influence and merely _eliminate_ variations. But, in both cases, the
outer conditions are supposed to bring about a precise adjustment of the
organism to its circumstances. Both parties, then, will attempt to
explain mechanically, by adaptation to similar conditions, the
similarities of structure which we think are the strongest argument
against mechanism. So we must at once indicate in a general way, before
passing to the detail, why explanations from "adaptation" seem to us
insufficient.

Let us first remark that, of the two hypotheses just described, the
latter is the only one which is not equivocal. The Darwinian idea of
adaptation by automati