cture 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 {488} 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 others; it
follows, that the amount of organic change in the fossils of consecutive
formations probably serves as a fair measure of the lapse of actual time. A
number of species, however, keeping in a body might remain for a long
period unchanged, whilst within this same period, s