, we may feel assured that they have both descended from
the same or nearly similar parents, and are therefore in that degree
closely related. Thus, community in embryonic structure reveals
community of descent. It will reveal this community of descent, however
much the structure of the adult may have been modified and obscured; we
have seen, for instance, that cirripedes can at once be recognised by
their larvae as belonging to the great class of crustaceans. As the
embryonic state of each species and group of species partially shows us
the structure of their less modified ancient progenitors, we can clearly
see why ancient and extinct forms of life should resemble the embryos of
their descendants,--our existing species. Agassiz believes this to be a
law of nature; but I am bound to confess that I only hope to see the
law hereafter proved true. It can be proved true in those cases alone in
which the ancient state, now supposed to be represented in many embryos,
has not been obliterated, either by the successive variations in a long
course of modification having supervened at a very early age, or by the
variations having been inherited at an earlier period than that at which
they first appeared. It should also be borne in mind, that the supposed
law of resemblance of ancient forms of life to the embryonic stages of
recent forms, may be true, but yet, owing to the geological record not
extending far enough back in time, may remain for a long period, or for
ever, incapable of demonstration.

Thus, as it seems to me, the leading facts in embryology, which are
second in importance to none in natural history, are explained on the
principle of slight modifications not appearing, in the many descendants
from some one ancient progenitor, at a very early period in the life of
each, though perhaps caused at the earliest, and being inherited at a
corresponding not early period. Embryology rises greatly in interest,
when we thus look at the embryo as a picture, more or less obscured, of
the common parent-form of each great class of animals.

RUDIMENTARY, ATROPHIED, OR ABORTED ORGANS.

Organs or parts in this strange condition, bearing the stamp of
inutility, are extremely common throughout nature. For instance,
rudimentary mammae are very general in the males of mammals: I presume
that the "bastard-wing" in birds may be safely considered as a digit
in a rudimentary state: in very many snakes one lobe of the lungs is
rudimentary;