zoölogy and botany had attained the point where there were
considerable treatises on those subjects. It was not, however, until a
little more than a century ago that men began accurately to describe
and classify these species of the organic world. Since the time of
Linnæus the growth of our knowledge has gone forward with amazing
swiftness. Within a century we have come to know perhaps a hundred
times as much concerning these creatures as was learned in all the
earlier ages. This knowledge is divisible into two main branches: in
one the inquirers have taken account of the different species, genera,
families, orders, and classes of living forms with such effect that
they have shown the existence at the present time of many hundred
thousand distinct species, the vast assemblage being arranged in a
classification which shows something as to the relationship which the
forms bear to each other, and furthermore that the kinds now living
have not been long in existence, but that at each stage in the history
of the earth another assemblage of species peopled the waters and the
lands.

At first naturalists concerned themselves only with the external forms
of living creatures; but they soon came to perceive that the way in
which these organisms worked, their physiology, in a word, afforded
matters for extended inquiry. These researches have developed the
science of physiology, or the laws of bodily action, on many accounts
the most modern and extensive of our new acquisitions of natural
learning. Through these studies we have come to know something of the
laws or principles by which life is handed on from generation to
generation, and by which the gradations of structure have been
advanced from the simple creatures which appear like bits of animated
jelly to the body and mind of man.