light of the Butterfly,--the quivering poise of the
Humming-Bird,--the arrow-like shooting of the Squid through the water,
--the slow crawling of the Snail on the land,--the sideway movement
of the Sand-Crab,--the backward walk of the Crawfish,--the almost
imperceptible gliding of the Sea-Anemone over the rock,--the graceful,
rapid motion of the Pleurobrachia, with its endless change of curve and
spiral. In short, every Family of animals has its characteristic action
and its peculiar voice; and yet so little is this endless variety
of rhythm and cadence both of motion and sound in the organic world
understood, that we lack words to express one-half its richness and
beauty.

IX.

The well-known meaning of the words _generic_ and _specific_ may serve,
in the absence of a more precise definition, to express the relative
importance of those groups of animals called Genera and Species in our
scientific systems. The Genus is the more comprehensive of the two kinds
of groups, while the Species is the most precisely defined, or at least
the most easily recognized, of all the divisions of the Animal Kingdom.
But neither the term Genus nor Species has always been taken in the same
sense. Genus especially has varied in its acceptation, from the time
when Aristotle applied it indiscriminately to any kind of comprehensive
group, from the Classes down to what we commonly call Genera, till the
present day. But we have already seen, that, instead of calling all the
various kinds of more comprehensive divisions by the name of Genera,
modern science has applied special names to each of them, and we have
now Families, Orders, Classes, and Branches above Genera proper. If
the foregoing discussion upon the nature of these groups is based upon
trustworthy principles, we must admit that they are all founded upon
distinct categories of characters,--the primary divisions, or the
Branches, on plan of structure, the Classes upon the manner of its
execution, the Orders upon the greater or less complication of a given
mode of execution, the Families upon form; and it now remains to be
ascertained whether Genera also exist in Nature, and by what kind of
characteristics they may be distinguished. Taking the practice of the
ablest naturalists in discriminating Genera as a guide in our estimation
of their true nature, we must, nevertheless, remember that even now,
while their classifications of the more comprehensive groups usually
agree, they di