ve hypothesis would modify this view, in assigning to species
only a temporary fixity, is obvious. Yet, if naturalists adopt this
hypothesis, they will still retain Jussieu's definition, which leaves
untouched the question as to how and when the "perennial successions"
were established. The practical question will only be, How much
difference between two sets of individuals entitles them to rank under
distinct species; and that is the practical question now, on whatever
theory. The theoretical question is--as stated at the beginning of
this long article--whether these specific lines were always as
distinct as now.

Mr. Agassiz has "lost no opportunity of urging the idea, that, while
species have no material existence, they yet exist as categories of
thought in the same way [and only in the same way] as genera,
families, orders, classes," etc. He "has taken the ground, that all
the natural divisions in the animal kingdom are primarily distinct,
founded upon different categories of characters, and that all exist in
the same way, that is, as categories of thought, embodied in
individual living forms. I have attempted to show that branches in the
animal kingdom are founded upon different plans of structure, and for
that very reason have embraced from the beginning representatives
between which there could be no community of origin; that classes are
founded upon different modes of execution of these plans, and
therefore they also embrace representatives which could have no
community of origin; that orders represent the different degrees of
complication in the mode of execution of each class, and therefore
embrace representatives which could not have a community of origin any
more than the members of different classes or branches; that families
are founded upon different patterns of form, and embrace
representatives equally independent in their origin; that genera are
founded upon ultimate peculiarities of structure, embracing
representatives which, from the very nature of their peculiarities,
could have no community of origin; and that, finally, species are
based upon relations and proportions that exclude, as much as all the
preceding distinctions, the idea of a common descent.

"As the community of characters among the beings belonging to these
different categories arises from the intellectual connection which
shows them to be categories of thought, they cannot be the result of a
gradual material differentiation of the o