ery difficulty; it was that
of the correlation of forms in organic beings, by means of which
each kind of organism can with exactitude be recognized by every
fragment of each of its parts.--Every organized being," he adds,
"forms an entire system, unique and closed, whose organs mutually
correspond, and concur in the same definite action by a reciprocal
reaction. Hence none of these parts can change without the other
being also modified, and consequently each of them, taken
separately, indicates and produces (_donne_) all the others.

"A claw, a shoulder-blade, a condyle, a leg or arm-bone, or any
other bone separately considered, enables us to discover the kind of
teeth to which they have belonged; so also reciprocally we may
determine the form of the other bones from the teeth. Thus,
commencing our investigation by a careful survey of any one bone by
itself, a person who is sufficiently master of the laws of organic
structure can reconstruct the entire animal. The smallest facet of
bone, the smallest apophysis, has a determinate character, relative
to the class, the order, the genus, and the species to which it
belongs, so that even when one has only the extremity of a
well-preserved bone, he can, with careful examination, assisted by
analogy and exact comparison, determine all these things as surely
as if he had before him the entire animal."

Cuvier adds that he has enjoyed every kind of advantage for such
investigations owing to his fortunate situation in the Museum of Natural
History, and that by assiduous researches for nearly thirty years[97]
he has collected skeletons of all the genera and sub-genera of
quadrupeds, with those of many species in certain genera, and several
individuals of certain species. With such means it was easy for him to
multiply his comparisons, and to verify in all their details the
applications of his laws.

Such is the famous law of correlation of parts, of Cuvier. It could be
easily understood by the layman, and its enunciation added vastly to the
popular reputation and prestige of the young science of comparative
anatomy.[98] In his time, and applied to the forms occurring in the
Paris Basin, it was a most valuable, ingenious, and yet obvious method,
and even now is the principal rule the palaeontologist follows in
identifying fragments of fossils of any class. But it has its
limitations, and it goes without saying that the more complete th