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.
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