Here we meet with a great difficulty. Hitherto varieties and subspecies
have never been clearly defined, or when they have been, it was [225]
not by physiological, but only by morphological research. And the claims
of these two great lines of inquiry are obviously very diverging.
Morphological or comparative studies need a material standard, by which
it may be readily decided whether certain groups of animals and plants
are to be described or de-nominated as species, as subspecies or as
varieties. To get at the inner nature of the differences is in most
cases impossible, but a decision must be made. The physiological line of
inquiry has more time at its disposal; it calls for no haste. Its
experiments ordinarily cover years, and a conclusion is only to be
reached after long and often weary trials. There is no making a decision
on any matter until all doubtful points have been cleared up. Of course,
large groups of facts remain uncertain, awaiting a closer inquiry, and
the teacher is constrained to rely on the few known instances of
thoroughly investigated cases. These alone are safe guides, and it seems
far better to trust to them and to make use of them for the construction
of sharp conceptions, which may help us to point out the lines of
inquiry which are still open.
Leaving aside all such divisions and definitions, as were stamped with
the name of provisional species and varieties by the great systematist,
[226] Alphonse De Candolle, we may now try to give the proofs of our
assertion, by using only those instances that have been thoroughly
tested in every way.
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