"Happily comparative anatomy possessed a principle which, well developed, was capable of overcoming every 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.