e." On Lagerstroemia as comparable with the
Melastomads see Letter 689.) Fritz Muller's theory with regard to
the Melastomads and a number of analogous cases in other genera are
discussed in H. Muller's article in "Kosmos" (620/3. "Kosmos," XIII.,
1883, page 241.), where the literature is given. F. Muller's theory is
that in Heeria the yellow anthers serve merely as a means of attracting
pollen-collecting bees, while the longer stamens with purple or crimson
anthers supply pollen for fertilising purposes. If Muller is right the
pollen from the yellow anthers would not normally reach the stigma. The
increased vigour observed in the seedlings from the yellow anthers
would seem to resemble the good effect of a cross between different
individuals of the same species as worked out in "Cross and Self
Fertilisation," for it is difficult to believe that the pollen of the
purple anthers has become, by adaptation, less effective than that
of the yellow anthers. In the letters here given there is some
contradiction between the statements as to the position of the two
sets of stamens in relation to the sepals. According to Eichler
("Bluthendiagramme, II., page 482) the longer stamens may be either
epipetalous or episepalous in this family.

The work on the Melastomads is of such intrinsic importance that we have
thought it right to give the correspondence in considerable detail; we
have done so in spite of the fact that Darwin arrived at no
definite conclusion, and in spite of an element of confusion and
unsatisfactoriness in the series of letters. This applies also to Letter
629, written after Darwin had learned Fritz Muller's theory, which is
obscured by some errors or slips of the pen.)

LETTER 620. TO G. BENTHAM. Down, February 3rd [1862?]

As you so kindly helped me before on dimorphism, will you forgive me
begging for a little further information, if in your power to give it?
The case is that of the Melastomads with eight stamens, on which I have
been experimenting. I am perplexed by opposed statements: Lindley says
the stamens which face the petals are sterile; Wallich says in Oxyspora
paniculata that the stamens which face the sepals are destitute of
pollen; I find plenty of apparently good pollen in both sets of stamens
in Heterocentron [Heeria], Monochoetum, and Centradenia. Can you throw
any light on this? But there is another point on which I am more anxious
for information. Please look at the enclosed miserable dia