of little importance. Darwin did
not deny the occurrence of self-fertilisation, even for a long series
of generations; his law states only that "Nature abhors PERPETUAL
self-fertilisation." (It is impossible (as has been attempted) to
express Darwin's point of view in a single sentence, such as H.
Muller's statement of the "Knight-Darwin law." The conditions of life in
organisms are so various and complex that laws, such as are formulated
in physics and chemistry, can hardly be conceived.) An exception to
this rule would therefore occur only in the case of plants in which the
possibility of cross-pollination is excluded. Some of the plants with
cleistogamous flowers might afford examples of such cases. We have
already seen, however, that such a case has not as yet been shown to
occur. Burck believed that he had found an instance in certain
tropical plants (Anonaceae, Myrmecodia) of the complete exclusion of
cross-fertilisation. The flowers of these plants, in which, however,--in
contrast to the cleistogamous flowers--the corolla is well developed,
remain closed and fruit is produced.

Loew (E. Loew, "Bemerkungen zu Burck... ", "Biolog. Centralbl." XXVI.
(1906).) has shown that cases occur in which cross-fertilisation may be
effected even in these "cleistopetalous" flowers: humming birds visit
the permanently closed flowers of certain species of Nidularium and
transport the pollen. The fact that the formation of hybrids may occur
as the result of this shows that pollination may be accomplished.

The existence of plants for which self-pollination is of greater
importance than it is for others is by no means contradictory to
Darwin's view. Self-fertilisation is, for example, of greater importance
for annuals than for perennials as without it seeds might fail to be
produced. Even in the case of annual plants with small inconspicuous
flowers in which self-fertilisation usually occurs, such as Senecio
vulgaris, Capsella bursa-pastoris and Stellaria media, A. Bateson (Anna
Bateson, "The effects of cross-fertilisation on inconspicuous
flowers", "Annals of Botany", Vol. I. 1888, page 255.) found that
cross-fertilisation gave a beneficial result, although only in a slight
degree. If the favourable effects of sexual reproduction, according to
Darwin's view, are correlated with change of environment, it is quite
possible that this is of less importance in plants which die after
ripening their seeds ("hapaxanthic") and which in any