one great exception of
fertility, the most striking accordance in all other respects; namely, in
the laws of their resemblance to their two parents, in their tendency to
reversion, in their variability, and in being absorbed through repeated
crosses by either parent-form.

Since arriving at the foregoing conclusions, condensed from my former work,
I have been led to investigate a subject which throws considerable light on
hybridism, namely, the fertility of {181} reciprocally dimorphic and
trimorphic plants, when illegitimately united. I have had occasion several
times to allude to these plants, and I may here give a brief abstract[441]
of my observations. 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; both with
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, there are
altogether six sets of stamens and three kinds of pistils. These organs are
so proportioned in length to each other that, in any two of the forms, half
the stamens in each 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 the other 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 species six unions are legitimate or fully fertile, and twelve
are illegitimate or more or less infertile.

The infertility which may be observed in various dimorphic and trimorphic
plants, when they are illegitimately fertilised, that is, by pollen taken
from stamens not corresponding in height with the pistil, differs much in
degree, up to absolute and utter sterility; just in the same manner as
occurs in crossing distinct species. As the degree of sterility in the
latter case depends in an eminent degree on the conditions of life being
more or les