mpler when we have a definite object in view. We possess no
pedigree or armorial bearings; and we have to discover and trace
the many diverging lines of descent in our natural genealogies, by
characters of any kind which have long been inherited. Rudimentary
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 often 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 birth-place; 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 between 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 occurrence of favourable 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 do not include many identical
species, by the general succession of the forms of life. As species are
produced and exterminated by slowly acting and still existing causes,
and not by miraculous acts of creation; and as the most important of all
causes of organic change is one which is almost indep