CHAPTER XIV. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY -- EMBRYOLOGY -- RUDIMENTARY ORGANS.
5. DEVELOPMENT AND EMBRYOLOGY.
This is one of the most important subjects in the whole round of natural
history. The metamorphoses of insects, with which every one is familiar,
are generally effected abruptly by a few stages; but the transformations
are in reality numerous and gradual, though concealed. A certain
ephemerous insect (Chloeon) during its development, moults, as shown by Sir
J. Lubbock, above twenty times, and each time undergoes a certain amount of
change; and in this case we see the act of metamorphosis performed in a
primary and gradual manner. Many insects, and especially certain
crustaceans, show us what wonderful changes of structure can be effected
during development. Such changes, however, reach their acme in the so-
called alternate generations of some of the lower animals. It is, for
instance, an astonishing fact that a delicate branching coralline, studded
with polypi, and attached to a submarine rock, should produce, first by
budding and then by transverse division, a host of huge floating jelly-
fishes; and that these should produce eggs, from which are hatched swimming
animalcules, which attach themselves to rocks and become developed into
branching corallines; and so on in an endless cycle. The belief in the
essential identity of the process of alternate generation and of ordinary
metamorphosis has been greatly strengthened by Wagner's discovery of the
larva or maggot of a fly, namely the Cecidomyia, producing asexually other
larvae, and these others, which finally are developed into mature males and
females, propagating their kind in the ordinary manner by eggs.
It may be worth notice that when Wagner's remarkable discovery was first
announced, I was asked how was it possible to account for the larvae of
this fly having acquired the power of a sexual reproduction. As long as
the case remained unique no answer could be given. But already Grimm has
shown that another fly, a Chironomus, reproduces itself in nearly the same
manner, and he believes that this occurs frequently in the order. It is
the pupa, and not the larva, of the Chironomus which has this power; and
Grimm further shows that this case, to a certain extent, "unites that of
the Cecidomyia with the parthenogenesis of the Coccidae;" the term
parthenogenesis implying that the mature females of the Coccidae are
capable of producing fertile eggs without the concourse of the male.
Certain animals belonging to several classes are now known to have the
power of ordinary reproduction at an unusually early age; and we have only
to accelerate parthenogenetic reproduction by gradual steps to an earlier
and earlier age--Chironomus showing us an almost exactly intermediate
stage, viz., that of the pupa--and we can perhaps account for the
marvellous case of the Cecidomyia.