Chap. XIII. 
EMBEYOLOGY. 
449 
and in so far it reveals the structure of its progenitor. 
In two groups of animals, however much they may at 
present differ from each other in structure and habits, if 
they pass through the same or similar embryonic stages, 
we may feel assured that they have both descended 
from the same or nearly similar parents, and are there¬ 
fore 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 progeni¬ 
tors, we can clearly see why ancient and extinct forms 
of life should resemble the embryos of their descend¬ 
ants,—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 existing em¬ 
bryos, has not been obliterated, either by the successive 
variations in a long course of modification having super¬ 
vened 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 extend¬ 
ing 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 embryo- 
logy, which are second in importance to none in natural 
history, are explained on the principle of slight modifi- 
