heart.[110]

True homologies can be established between materials of organisation but
not always between organs, which may be composed of different
"materials."

Almost as a corollary to this comes the further view that form is of
little importance in determining homologies. An organ is essentially an
instrument for doing a particular kind of work, and its form is
determined by its function. Organs which perform the same function are
usually similar in form though the elementary materials composing them
may be different. This is seen in many cases of convergence. Organs,
therefore, which perform the same function and are similar in external
form are not necessary homologous. Conversely, the same complex of
materials, say a fore limb, may take on the most varied shapes according
as the function of the organ changes--but homology remains though form
changes. Accordingly, form is one of the least important elements to be
considered in determining a homology. "Nature," he wrote in one of his
early papers, "tends to repeat the same organs in the same number and in
the same relations, and varies to infinity only their form. In
accordance with this principle I shall have to draw my conclusions, in
the determining the bones of the fish's skull, not from a consideration
of their form, but from a consideration of their connections."[111]

Again, after comparing a vertebra of the Aurochs with an abdominal
segment of the crab, he says, "I have insisted upon an identity which
has extended to the least important relation of all, that of form."[112]

Geoffroy's morphological units or materials of organisation were in the
case of the skeleton--with which his researches principally deal--the
single bones. But the interesting point is that he sought his
skeleton-units in the embryo, and considered each separate centre of
ossification as a separate bone. Coalescence of bones originally
separate is one of the most usual events in development, and it is an
occurrence which, more than any other, tends to obscure homologies.
Because of its coalescence with the maxillaries, the intermaxillary in
man was not discovered until Vicq d'Azyr and Goethe found it separate in
the embryo. Apparently quite independently of Goethe, Geoffroy hit upon
this plan of seeking in the embryo the primary elements or materials of
organisation. In an early paper on the skull of Vertebrates,[113] where he
is concerned with showing that each bone of the fish's skul