The common view of species is, that, although they are
generalizations, yet they have a direct objective ground in Nature,
which genera, orders, etc., have not. According to the succinct
definition of Jussieu,--and that of Linnaeus is identical in
meaning,--a species is the perennial succession of similar individuals
in continued generations. The species is the chain of which the
individuals are the links. The sum of the genealogically connected
similar individuals constitutes the species, which thus has an
actuality and ground of distinction not shared by genera and other
groups which were not supposed to be genealogically connected. How a
derivative 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.
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