I am fully persuaded that this double
parallelism is by no means an accident or an illusion. He who is able to
explain why the elephant, and a multitude of other animals, are incapable
of breeding when kept under only partial confinement in their native
country, will be able to explain the primary cause of hybrids being so
generally sterile. He will at the same time be able to explain how it is
that the races of some of our domesticated animals, which have often been
subjected to new and not uniform conditions, are quite fertile together,
although they are descended from distinct species, which would probably
have been sterile if aboriginally crossed. The above two parallel series
of facts seem to be connected together by some common but unknown bond,
which is essentially related to the principle of life; this principle,
according to Mr. Herbert Spencer, being that life depends on, or consists
in, the incessant action and reaction of various forces, which, as
throughout nature, are always tending towards an equilibrium; and when this
tendency is slightly disturbed by any change, the vital forces gain in
power.
RECIPROCAL DIMORPHISM AND TRIMORPHISM.
This subject may be here briefly discussed, and will be found to throw some
light on hybridism. Several plants belonging to distinct orders present
two forms, which exist in about equal numbers and which differ in no
respect except in their reproductive organs; one form having a long pistil
with short stamens, the other a short pistil with long stamens; the two
having differently sized pollen-grains.
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