cs and vertebrates separated from their common parent-stem
long before the appearance of an eye so complex as that of the Pecten.
Whence, then, the structural analogy?

Let us question on this point the two opposed systems of evolutionist
explanation in turn--the hypothesis of purely accidental variations, and
that of a variation directed in a definite way under the influence of
external conditions.

The first, as is well known, is presented to-day in two quite different
forms. Darwin spoke of very slight variations being accumulated by
natural selection. He was not ignorant of the facts of sudden variation;
but he thought these "sports," as he called them, were only
monstrosities incapable of perpetuating themselves; and he accounted for
the genesis of species by an accumulation of _insensible_
variations.[26] Such is still the opinion of many naturalists. It is
tending, however, to give way to the opposite idea that a new species
comes into being all at once by the simultaneous appearance of several
new characters, all somewhat different from the previous ones. This
latter hypothesis, already proposed by various authors, notably by
Bateson in a remarkable book,[27] has become deeply significant and
acquired great force since the striking experiments of Hugo de Vries.
This botanist, working on the _OEnothera Lamarckiana_, obtained at the
end of a few generations a certain number of new species. The theory he
deduces from his experiments is of the highest interest. Species pass
through alternate periods of stability and transformation. When the
period of "mutability" occurs, unexpected forms spring forth in a great
number of different directions.[28]--We will not attempt to take sides
between this hypothesis and that of insensible variations. Indeed,
perhaps both are partly true. We wish merely to point out that if the
variations invoked are accidental, they do not, whether small or great,
account for a similarity of structure such as we have cited.

Let us assume, to begin with, the Darwinian theory of insensible
variations, and suppose the occurrence of small differences due to
chance, and continually accumulating. It must not be forgotten that all
the parts of an organism are necessarily coordinated. Whether the
function be the effect of the organ or its cause, it matters little; one
point is certain--the organ will be of no use and will not give
selection a hold unless it functions. However the minute structure of