"
Its nine sections treat successively of the great types or branches
of the animal kingdom, of classes, orders, families, genera, species,
other natural divisions, successive development of characters, and
close with some very significant conclusions on the importance of
the study of classification.
Mr. Agassiz has attempted to give definiteness to the terms above
enumerated, which have been used with various significance, by
limiting each one of them to covering a single category of natural
relationship. Thus:--
_Branches_ or _types_ are characterized by their plan of structure.
_Classes_, by the manner in which that plan is executed, so far as
ways and means are concerned.
_Orders_, by the degrees of complication of that structure.
_Families_, by their form, so far as determined by structure.
_Genera_, by the details of the execution in special parts.
_Species_, by the relations of individuals to one another and to
the world in which they live, as well as by the proportions of their
parts, their ornamentation, etc.
"And yet there are other natural divisions which must be acknowledged
in a natural zooelogical system; but these are not to be traced so
uniformly in all classes as the former,--they are, in reality, only
limitations of the other kinds of divisions.
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