The Life and Letters of Darwin, Volume 2
'Different Forms Of Flowers On Plants Of The Same
[The volume bearing the above title was published in 1877, and was dedicated by the
author to Professor Asa Gray, "as a small tribute of respect and affection." It consists of
certain earlier papers re-edited, with the addition of a quantity of new matter. The
subjects treated in the book are:--
1. Heterostyled Plants.
2. Polygamous, Dioecious, and Gynodioecious Plants.
3. Cleistogamic Flowers.
The nature of heterostyled plants may be illustrated in the primrose, one of the best
known examples of the class. If a number of primroses be gathered, it will be found that
some plants yield nothing but "pin-eyed" flowers, in which the style (or organ for the
transmission of the pollen to the ovule) is long, while the others yield only "thrum-eyed"
flowers with short styles. Thus primroses are divided into two sets or castes differing
structurally from each other. My father showed that they also differ sexually, and that in
fact the bond between the two castes more nearly resembles that between separate sexes
than any other known relationship. Thus for example a long-styled primrose, though it
can be fertilised by its own pollen, is not FULLY fertile unless it is impregnated by the
pollen of a short-styled flower. Heterostyled plants are comparable to hermaphrodite
animals, such as snails, which require the concourse of two individuals, although each
possesses both the sexual elements. The difference is that in the case of the primrose it is
PERFECT FERTILITY, and not simply FERTILITY, that depends on the mutual action
of the two sets of individuals.
The work on heterostyled plants has a special bearing, to which the author attached much
importance, on the problem of origin of species. (See 'Autobiography,' volume i.)
He found that a wonderfully close parallelism exists between hybridisation and certain
forms of fertilisation among heterostyled plants. So that it is hardly an exaggeration to
say that the "illegitimately" reared seedlings are hybrids, although both their parents
belong to identically the same species. In a letter to Professor Huxley, my father writes as
if his researches on heterostyled plants tended to make him believe that sterility is a
selected or acquired quality. But in his later publications, e.g. in the sixth edition of the
'Origin,' he adheres to the belief that sterility is an incidental rather than a selected
quality. The result of his work on heterostyled plants is of importance as showing that
sterility is no test of specific distinctness, and that it depends on differentiation of the
sexual elements which is independent of any racial difference. I imagine that it was his
instinctive love of making out a difficulty which to a great extent kept him at work so