organs will speak infallibly with respect to the nature of long-lost
structures. Species and groups of species, which are called aberrant,
and which may fancifully be called living fossils, will aid us in
forming a picture of the ancient forms of life. Embryology will reveal
to us the structure, in some degree obscured, of the prototypes of each
great class.
When we can feel assured that all the individuals of the same species,
and all the closely allied species of most genera, have within a not
very remote period descended
from one parent, and have migrated from some one birthplace; and when
we better know the many means of migration, then, by the light which
geology now throws, and will continue to throw, on former changes of
climate and of the level of the land, we shall surely be enabled to
trace in an admirable manner the former migrations of the inhabitants
of the whole world. Even at present, by comparing the differences of
the inhabitants of the sea on the opposite sides of a continent, and
the nature of the various inhabitants of that continent in relation to
their apparent means of immigration, some light can be thrown on
ancient geography.
The noble science of Geology loses glory from the extreme imperfection
of the record. The crust of the earth with its embedded remains must
not be looked at as a well-filled museum, but as a poor collection made
at hazard and at rare intervals. The accumulation of each great
fossiliferous formation will be recognised as having depended on an
unusual concurrence of circumstances, and the blank intervals between
the successive stages as having been of vast duration. But we shall be
able to gauge with some security the duration of these intervals by a
comparison of the preceding and succeeding organic forms. We must be
cautious in attempting to correlate as strictly contemporaneous two
formations, which include few identical species, by the general
succession of their forms of life. As species are produced and
exterminated by slowly acting and still existing causes, and not by
miraculous acts of creation and by catastrophes; and as the most
important of all causes of organic change is one which is almost
independent of altered and perhaps suddenly altered physical
conditions, namely, the mutual relation of organism to organism, the
improvement of one being entailing the improvement or the extermination
of
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