ernal. "The minuteness of the
episternals and xiphisternals might be imputed to this gigantic piece
diverting to its own profit the nutritive fluid, since the bigger it is
the smaller these are."[116]

One has constantly to remember in dealing with Geoffroy's theories that
he was not an evolutionist, but purely a morphologist. It is therefore,
perhaps, to ask too much to require of him an explanation of the causes
of diversity. The morphologist describes, classifies, generalises; he
does not seek for causes. But we must leave this question aside in order
to discuss how far Geoffroy's theory of the unity of plan and
composition fits the facts. As Geoffroy himself admitted on several
occasions, his theory was an _a priori_ one, a theory hit upon by hasty
induction, then erected into a principle and imposed upon the facts. No
more than Goethe did he extract his principle from a sufficient mass of
data.

Now he found his theory to be in its pure form unworkable; he found, for
example, that the skeleton of fishes could not be compared directly,
bone for bone, with the skeleton of higher Vertebrates; he had to admit
differences of position of whole sets of organs in the two groups, he
had to admit various _metastases_, before he could bring the skeleton of
fish into line. And these metastases are due to functional
requirements--for example, the forward position of sternum and thoracic
organs in fish is an adaptation to swimming.

So he does not so much demonstrate the unity of plan of whole organisms
as the unity of plan of particular corresponding parts of them. Thus he
does not prove or attempt to prove that Articulates are in all points
like Vertebrates, but simply that their skeleton is built upon the same
plan as that of Vertebrates. The rest of the organs, while still
comparable with the organs of Vertebrates, stand in different relations
to the skeleton. An Articulate therefore, on his own showing, is not,
_as a whole_, built upon the same general structural plan as a
Vertebrate.

Further, he does not always remain true to his principles, for he does
not establish homologies of parts entirely by their connections but
sometimes by their functions as well. Thus the sternum, or rather the
complex of sternal elements, is defined and discovered in particular
cases not by its connections only but also by its functions. The
framework of the gills is homologised part by part with the framework of
the lungs, not because th