This great paleontologist, in the course of his ichthyological
researches, was led to perceive that the arrangement by Cuvier
according to organs did not fulfil its purpose with regard to fossil
fishes, because in the lapse of ages the characteristics of their
structures were destroyed. He therefore adopted the only other
remaining plan, and studied the tissues, which, being less complex
than the organs, are oftener found intact. The result was the very
remarkable discovery, that the tegumentary membrane of fishes is so
intimately connected with their organisation, that if the whole of the
fish has perished except this membrane, it is practicable, by noting
its characteristics, to reconstruct the animal in its most essential
parts. Of the value of this principle of harmony, some idea may be
formed from the circumstance, that on it Agassiz has based the whole
of that celebrated classification of which he is the sole author, and
by which fossil ichthyology has for the first time assumed a precise
and definite shape. How essential its study is to the geologist appears
from the remark of Sir Roderick Murchison, that “fossil fishes have
every where proved the most exact chronometer of the age of rocks.”