Two hypotheses divide the scientific world, very unequally, upon the
origin of the existing diversity of the plants and animals which
surround us. One assumes that the actual kinds are primordial; the
other, that they are derivative. One, that all kinds originated
supernaturally and directly as such, and have continued unchanged in
the order of Nature; the other, that the present kinds appeared in
some sort of genealogical connection with other and earlier kinds,
that they became what they now are in the course of time and in the
order of Nature.
Or, bringing in the word _species_, which is well defined as "the
perennial succession of individuals," commonly of very like
individuals,--as a close corporation of individuals perpetuated by
generation, instead of election,--and reducing the question to
mathematical simplicity of statement: species are lines of individuals
coming down from the past and running on to the future,--lines
receding, therefore, from our view in either direction. Within our
limited view they appear to be parallel lines, as a general thing
neither approaching to nor diverging from each other. The first
hypothesis assumes that they were parallel from the unknown beginning
and will be to the unknown end. The second hypothesis assumes that the
apparent parallelism is not real and complete, at least aboriginally,
but approximate or temporary; that we should find the lines convergent
in the past, if we could trace them far enough; that some of them, if
produced back, would fall into certain fragments of lines, which have
left traces in the past, lying not exactly in the same direction, and
these farther back into others to which they are equally unparallel.
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