er than Cuvier. They laid great
store by homological resemblances, and dismissed analogies of structure
as of little interest. They were singularly unwilling to admit the
existence of convergence or of parallel evolution, and they held very
firmly the distinctively Geoffroyan view that Nature is so limited by
the unity of composition that she can and does form no new organs.

By no one has this underlying principle of evolutionary morphology been
more explicitly recognised than by Hubrecht, who in his paper of 1887,
after summarising the points of resemblance between Nemertines and
Vertebrates which led him to assume a genetic connection between them,
writes as follows:--"At the base of all the speculations contained in
this chapter lies the conviction, so strongly insisted upon by Darwin,
that new combinations or organs do not appear by the action of natural
selection unless others have preceded, from which they are gradually
derived by a slow change and differentiation.

"That a notochord should develop out of the archenteric wall because a
supporting axis would be beneficial to the animal may be a teleological
assumption, but it is at the same time an evolutional heresy. It would
never be fruitful to try to connect the different variations offered,
_e.g._, by the nervous system throughout the animal kingdom, if similar
assumptions were admitted, for there would be then quite as much to say
for a repeated and independent origin of central nervous systems out of
indifferent epiblast just as required in each special case. These would
be steps that might bring us back a good way towards the doctrine of
independent creations. The remembrance of Darwin's, Huxley's, and
Gegenbaur's classical foundations, and of Balfour's and Weismann's
brilliant superstructures, ought to warn us away from these dangerous
regions" (p. 644).

This same prejudice lies at the root of the idea of _Functionswechsel_,
in spite of the general functional orientation of that idea.

Dohrn's constant assumption is that Nature makes shift with old organs
wherever possible, instead of forming new ones. He derives gill-slits
from segmental organs, fins and limbs from gills, ribs from gill-arches,
and so on, instead of admitting that these organs might quite as well
have arisen independently. He objects on principle to the origin of
organs _de novo_. Thus, rebutting the suggestion that certain organs
which are not found in the lower Vertebrates might ha