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.
We may, however, infer as probable from the consideration of dimorphic and
trimorphic plants, that the sterility of distinct species when crossed and
of their hybrid progeny, depends exclusively on the nature of their sexual
elements, and not on any difference in their structure or general
constitution. We are also led to this same conclusion by considering
reciprocal crosses, in which the male of one species cannot be united, or
can be united with great difficulty, with the female of a second species,
while the converse cross can be effected with perfect facility. That
excellent observer, Gartner, likewise concluded that species when crossed
are sterile owing to differences confined to their reproductive systems.
FERTILITY OF VARIETIES WHEN CROSSED, AND OF THEIR MONGREL OFFSPRING, NOT
UNIVERSAL.
It may be urged as an overwhelming argument that there must be some
essential distinction between species and varieties inasmuch as the latter,
however much they may differ from each other in external appearance, cross
with perfect facility, and yield perfectly fertile offspring. With some
exceptions, presently to be given, I fully admit that this is the rule.
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