tiae lie the only keys to the
most interesting riddles offered by the extinct animal world.

These assuredly are great and solid gains. Surely it is matter for no
small congratulation that in half a century (for palaeontology, though it
dawned earlier, came into full day only with Cuvier) a subordinate
branch of biology should have doubled the value and the interest of the
whole group of sciences to which it belongs.

But this is not all. Allied with geology, palaeontology has established
two laws of inestimable importance: the first, that one and the same
area of the earth's surface has been successively occupied by very
different kinds of living beings; the second, that the order of
succession established in one locality holds good, approximately, in
all.

The first of these laws is universal and irreversible; the second is an
induction from a vast number of observations, though it may possibly,
and even probably, have to admit of exceptions. As a consequence of the
second law, it follows that a peculiar relation frequently subsists
between series of strata, containing organic remains, in different
localities. The series resemble one another, not only in virtue of a
general resemblance of the organic remains in the two, but also in
virtue of a resemblance in the order and character of the serial
succession in each. There is a resemblance of arrangement; so that the
separate terms of each series, as well as the whole series, exhibit a
correspondence.

Succession implies time; the lower members of a series of sedimentary
rocks are certainly older than the upper; and when the notion of age was
once introduced as the equivalent of succession, it was no wonder that
correspondence in succession came to be looked upon as correspondence in
age, or "contemporaneity." And, indeed, so long as relative age only is
spoken of, correspondence in succession _is_ correspondence in age; it
is _relative_ contemporaneity.

But it would have been very much better for geology if so loose and
ambiguous a word as "contemporaneous" had been excluded from her
terminology, and if, in its stead, some term expressing similarity of
serial relation, and excluding the notion of time altogether, had been
employed to denote correspondence in position in two or more series of
strata.

In anatomy, where such correspondence of position has constantly to be
spoken of, it is denoted by the word "homology" and its derivatives; and
for Geology (whi