CHAPTER IX. HYBRIDISM.
5. RECIPROCAL DIMORPHISM AND TRIMORPHISM. (continued)
Altogether there is the closest identity in character and behaviour between
illegitimate plants and hybrids. It is hardly an exaggeration to maintain
that illegitimate plants are hybrids, produced within the limits of the
same species by the improper union of certain forms, while ordinary hybrids
are produced from an improper union between so-called distinct species. We
have also already seen that there is the closest similarity in all respects
between first illegitimate unions and first crosses between distinct
species. This will perhaps be made more fully apparent by an illustration;
we may suppose that a botanist found two well-marked varieties (and such
occur) of the long-styled form of the trimorphic Lythrum salicaria, and
that he determined to try by crossing whether they were specifically
distinct. He would find that they yielded only about one-fifth of the
proper number of seed, and that they behaved in all the other above
specified respects as if they had been two distinct species. But to make
the case sure, he would raise plants from his supposed hybridised seed, and
he would find that the seedlings were miserably dwarfed and utterly
sterile, and that they behaved in all other respects like ordinary hybrids.
He might then maintain that he had actually proved, in accordance with the
common view, that his two varieties were as good and as distinct species as
any in the world; but he would be completely mistaken.
The facts now given on dimorphic and trimorphic plants are important,
because they show us, first, that the physiological test of lessened
fertility, both in first crosses and in hybrids, is no safe criterion of
specific distinction; secondly, because we may conclude that there is some
unknown bond which connects the infertility of illegitimate unions with
that of their illegitimate offspring, and we are led to extend the same
view to first crosses and hybrids; thirdly, because we find, and this seems
to me of especial importance, that two or three forms of the same species
may exist and may differ in no respect whatever, either in structure or in
constitution, relatively to external conditions, and yet be sterile when
united in certain ways. For we must remember that it is the union of the
sexual elements of individuals of the same form, for instance, of two long-
styled forms, which results in sterility; while it is the union of the
sexual elements proper to two distinct forms which is fertile. Hence the
case appears at first sight exactly the reverse of what occurs, in the
ordinary unions of the individuals of the same species and with crosses
between distinct species. It is, however, doubtful whether this is really
so; but I will not enlarge on this obscure subject.
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