endages of Arthropods have become transformed into
jaws--their function as graspers of food has gradually prevailed over
their main function as walking limbs. In the evolution of Vertebrates
from Annelids the principle came into action in many connections--in the
formation of a new mouth from gill-slits, in the transformation of gills
into fins and limbs, of segmental organs into gill-slits, and so on.
Dohrn tells us that the principle of function-change was suggested to
him by Mivart's _Genesis of Species_ (1870), and he points out how it
enables a partial reply to be made to the dangerous objection raised
against the theory of natural selection that the first beginnings of new
organs are necessarily useless in the struggle for existence.

We may note in passing that a somewhat similar idea was later applied by
Kleinenberg to the explanation of some of the ancestral features of
development. He pointed out in his classical memoir on the embryology of
the Annelid _Lopadorhynchus_[400] that many embryonic organs seem to be
formed for the sole purpose of providing the necessary stimulus for the
development of the definitive organs. Thus the notochord is the
necessary forerunner of the vertebral column, cartilage the precursor of
bone. "From this point of view," he writes, "many rudimentary organs
appear in a different light. Their obstinate reappearance throughout
long phylogenetic series would be hard to understand were they really no
more than reminiscences of bygone and forgotten stages. Their
significance in the processes of individual development may in truth be
far greater than is generally recognised. When in the course of the
phylogeny they have played their part as intermediary organs
(_Vermittelungsorgane_) they assume the same function in the ontogeny.
Through the stimulus or by the aid of these organs, now become
rudimentary, the permanent parts of the embryo appear and are guided in
their development; when these have attained a certain degree of
independence, the intermediary organ, having played its part, may be
placed upon the retired list."[401]

Dohrn was well aware of the functional, or as he calls it, the
physiological, orientation of his principle, and he rightly regarded
this as one of its chief merits. He held that morphology became too
abstract and one-sided if it disregarded physiology completely; he saw
clearly that the evolution of function was quite as important a problem
as the evolution of for