ism 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. With trimorphic
plants there are three forms likewise differing in the lengths of their
pistils and stamens, in the size and colour of the pollen-grains, and
in some other respects; and as in each of the three forms there are two
sets of stamens, the three forms possess altogether six sets of stamens
and three kinds of pistils. These organs are so proportioned in length
to each other that half the stamens in two of the forms stand on a level
with the stigma of the third form. Now I have shown, and the result
has been confirmed by other observers, that in order to obtain full
fertility with these plants, it is necessary that the stigma of the
one form should be fertilised by pollen taken from the stamens of
corresponding height in another form. So that with dimorphic species
two unions, which may be called legitimate, are fully fertile; and
two, which may be called illegitimate, are more or less infertile. With
trimorphic