dred thousand years more to
the length of the experiment and to the force of their argument. As the
facts stand, it appears, that, while some tertiary forms are essentially
undistinguishable from existing ones, others are the same with a
difference, which is judged not to be specific or aboriginal, and yet
others show somewhat greater differences, such as are scientifically
expressed by calling them marked varieties, or else doubtful species;
while others, differing a little more, are confidently termed distinct,
but nearly related species. Now is not all this a question of degree,
of mere gradation of difference? Is it at all likely that these several
gradations came to be established in two totally different ways,--some
of them (though naturalists can't agree which) through natural
variation, or other secondary cause, and some by original creation,
without secondary cause? We have seen that the judicious Pictet answers
such questions as Darwin would have him do, in affirming, that, in all
probability, the nearly related species of two successive faunas were
materially connected, and that contemporaneous species, similarly
resembling each other, were not all created so, but have become so. This
is equivalent to saying that species (using the term as all naturalists
do and must continue to employ the word) have only a relative, not an
absolute fixity; that differences fully equivalent to what are held to
be specific may arise in the course of time, so that one species may at
length be naturally replaced by another species a good deal like it, or
may be diversified through variation or otherwise into two, three, or
more species, or forms as different as species. This concedes all that
Darwin has a right to ask, all that he can directly infer from evidence.
We must add that it affords a _locus standi_, more or less tenable, for
inferring more.

Here another geological consideration comes in to help on this
inference. The species of the later tertiary period for the most part
not only resembled those of our days, many of them so closely as to
suggest an absolute continuity, but, also occupied in general the same
regions that their relatives occupy now. The same may be said, though
less specially, of the earlier tertiary and of the later secondary; but
there is less and less localization of forms as we recede, yet some
localization even in palaeozoic times. While in the secondary period one
is struck with the similarity of