duce 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.

It has already been stated that various parts in the same individual,
which are exactly alike during an early embryonic period, become widely
different and serve for widely different purposes in the adult state. So
again it has been shown that generally the embryos of the most distinct
species belonging to the same class are closely similar, but become,
when fully developed, widely dissimilar. A better proof of this latter
fact cannot be given than the statement by Von Baer that "the embryos of
mammalia, of birds, lizards and snakes, probably also of chelonia, are
in the earliest states exceedingly like one another, both as a whole and
in the mode of development of their parts; so much so, in fact, that we
can often disting