ly learnt to think of the different animal
forms as developed one from another--and seemed, in some circles at
least, determined to forget that this metamorphosis could only be
conceptual" (p. 200). At the same time the theory of parallelism led men
to rehabilitate the outworn conception of the scale of beings, to
maintain that animals form one single series of increasing complexity, a
scale which the higher members must mount step by step in their
development--from which it followed that evolution, whether conceived as
an ideal or as an historical process, could take place only along one
line, could be only progressive or regressive. Not all the supporters of
the theory of parallelism held these extreme views, but conclusions of
this kind were natural and logical enough.

Von Baer had soon found in the course of his embryological studies that
the facts did not at all fit in with the doctrine of parallelism; the
developing chick, for example, was at a very early stage demonstrably a
Vertebrate, and did not recapitulate in its early stages the
organisation of a polyp, a worm or a mollusc. He had published his
doubts in 1823, but his final confutation of the theory of parallelism
is found in this Scholion.

If it were true, he says, that the essential thing in the development of
an animal is this repetition of lower organisations, then certain
deductions could be drawn, which one would expect to find confirmed in
Nature. The first deduction would be that no structures should appear in
the embryo of the higher animals that are not found in the lower
animals. But this is not confirmed by the facts--no adult among the
lower animals, for instance, has a yolk-sac like that of the chick
embryo. Again, if the law of parallelism were true, the mammalian embryo
would have to repeat the organisation of, among other groups, insects
and birds. But the embryo _in utero_ is surrounded by fluid and cannot
possibly breathe free air, so it cannot possibly repeat the structure of
either insects or birds, which are pre-eminently air-organisms.
Generally speaking, indeed, we find in all the higher embryos special
structures which adapt them to the very special conditions of their
development, and these we never find as permanent structures in the
lower animals. The supporters of the theory of parallelism might,
however, admit the existence of such special embryonic organs without
greatly prejudicing their case, for these temporary organs st