bly arose from the
coalescence of rows of parapodia. Dohrn assumed also that the primitive
Annelid ancestor must have possessed a notochord to give support in
swimming.

If Vertebrates arose from primitive Annelid ancestors, how account for
Amphioxus and the Ascidians, which seem to be the most primitive living
Vertebrates and yet show no particular annelidan affinities? Dohrn tries
to answer this awkward question by showing that these forms are not
primitive but degenerate. He points out first that Cyclostomes are
degenerate fish, half specialised and half degraded in adaptation to a
parasitic mode of life. He thinks that if an _Ammocoetes_ were to become
sexually mature and degenerate still further, forms would result which
would resemble Amphioxus, and ultimately, if the process of degeneration
went far enough, larval Ascidians. Amphioxus therefore might well be
considered an extremely simplified and degenerate Cyclostome, and the
ascidian larva the last term of this degeneration-series. Both Amphioxus
and the Ascidians would accordingly be descended from fish, instead of
fish being evolved from them.

Dohrn conceived that the transformation of the Annelid into the
Vertebrate took place mainly by reason of an important transforming
principle, which he calls the principle of function-change. Each organ,
Dohrn thinks, has besides its principal function a number of subsidiary
functions which only await an opportunity to become active. "The
transformation of an organ takes place by reason of the succession of
the functions which one and the same organ possesses. Each function is a
resultant of several components, of which one is the principal or
primary function, while the others are the subsidiary or secondary
functions. The weakening of the principal function and the strengthening
of a subsidiary function alters the total function; the subsidiary
function gradually becomes the chief function, the total function
becomes quite different, and the consequence of the whole process is the
transformation of the organ" (p. 60). Examples of function-change are
not difficult to find. Thus the stomach in most Vertebrates performs
both a chemical and a mechanical function, but in some forms a part of
it specialises in the mechanical side of the work and becomes a gizzard,
while the remaining part confines its energies to the secretion of the
gastric juice. So, too, it is through function-change that certain of
the ambulatory app