CHAPTER XIV. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY -- EMBRYOLOGY -- RUDIMENTARY ORGANS.
5. DEVELOPMENT AND EMBRYOLOGY. (continued)
On the other hand it is highly probable that with many animals the
embryonic or larval stages show us, more or less completely, the condition
of the progenitor of the whole group in its adult state. In the great
class of the Crustacea, forms wonderfully distinct from each other, namely,
suctorial parasites, cirripedes, entomostraca, and even the malacostraca,
appear at first as larvae under the nauplius-form; and as these larvae live
and feed in the open sea, and are not adapted for any peculiar habits of
life, and from other reasons assigned by Fritz Muller, it is probable that
at some very remote period an independent adult animal, resembling the
Nauplius, existed, and subsequently produced, along several divergent lines
of descent, the above-named great Crustacean groups. So again, it is
probable, from what we know of the embryos of mammals, birds, fishes and
reptiles, that these animals are the modified descendants of some ancient
progenitor, which was furnished in its adult state with branchiae, a swim-
bladder, four fin-like limbs, and a long tail, all fitted for an aquatic
life.
As all the organic beings, extinct and recent, which have ever lived, can
be arranged within a few great classes; and as all within each class have,
according to our theory, been connected together by fine gradations, the
best, and, if our collections were nearly perfect, the only possible
arrangement, would be genealogical; descent being the hidden bond of
connexion which naturalists have been seeking under the term of the Natural
System. On this view we can understand how it is that, in the eyes of most
naturalists, the structure of the embryo is even more important for
classification than that of the adult. In two or more groups of animals,
however much they may differ from each other in structure and habits in
their adult condition, if they pass through closely similar embryonic
stages, we may feel assured that they are all descended from one parent-
form, and are therefore closely related. Thus, community in embryonic
structure reveals community of descent; but dissimilarity in embryonic
development does not prove discommunity of descent, for in one of two
groups the developmental stages may have been suppressed, or may have been
so greatly modified through adaptation to new habits of life as to be no
longer recognisable. Even in groups, in which the adults have been
modified to an extreme degree, community of origin is often revealed by the
structure of the larvae; we have seen, for instance, that cirripedes,
though externally so like shell-fish, are at once known by their larvae to
belong to the great class of crustaceans. As the embryo often shows us
more or less plainly the structure of the less modified and ancient
progenitor of the group, we can see why ancient and extinct forms so often
resemble in their adult state the embryos of existing species of the same
class. Agassiz believes this to be a universal law of nature; and we may
hope hereafter to see the law proved true. It can, however, be proved true
only in those cases in which the ancient state of the progenitor of the
group has not been wholly obliterated, either by successive variations
having supervened at a very early period of growth, or by such variations
having been inherited at an earlier age than that at which they first
appeared. It should also be borne in mind, that the law 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. The
law will not strictly hold good in those cases in which an ancient form
became adapted in its larval state to some special line of life, and
transmitted the same larval state to a whole group of descendants; for such
larval state will not resemble any still more ancient form in its adult
state.