l has its
homologue in the skull of higher Vertebrates, he is faced with the
difficulty that the skull of the fish has more bones than the skull of
higher Vertebrates. "Having had the inspiration," he writes, "to reckon
as many bones as there are distinct centres of ossification, and having
made a consistent trial of this method, I have been able to appreciate
the correctness of the idea: fish, in their earliest stages, are in the
same conditions relatively to their development as the foetuses of
mammals, and hence bear out the theory" (p. 344). So, too, in dealing
with the homologies of the sternal elements (_supra_, p. 57) he treats
as separate bones the "annexes" of the sternum in birds, though these
are separate only in the young.

If the same materials of organisation are present in all animals, and if
they are arranged always in the same positions relatively to one
another, how does it come about that animal forms are so varied, what
explanation can be offered of the diversities of organic structure?
Geoffroy's main answer to this question is his _Loi de balancement_. The
law was enunciated by him already in 1807.[114] We take the following
quotation, which represents his thought most nearly, from the _Cours de
l'histoire naturelle des Mammiferes_ (1829). "According to our manner of
regarding the organisation of mammals, there is only a single animal
modified by the inverse reciprocal variation of all or some of its
parts. Now, from the fact that there is only one single general animal,
it follows that for each section of its components or for each of its
organs there is available only a given quantity of formative materials.
Now suppose that the distribution of these materials has not been made
in such a way as to ensure an exact equilibrium between all the parts
concerned, one organ will get more than its share, another less. My law
of the compensation of organs is founded on these principles" (i.,
_Lecon_ 16, p. 12). "The atrophy of one organ turns to the profit of
another; and the reason why this cannot be otherwise is simple, it is
because there is not an unlimited supply of the substance required for
each special purpose."[115] The nutritive material available is limited
for each species; if one part gets more than its share the other parts
must get less--that is all the law means. As an example, take the
minuteness of the episternals and xiphisternals in birds, as contrasted
with the huge size of the entost