e given in "Animals and Plants," Edition II., Volume
I., page 408.) I enclose varieties of maize from Asa Gray. Pray do not
thank me for trusting you; the thanks ought to go the other way. I
felt a conviction after your first letter that you were a real lover of
Natural History.

If you can advance good evidence showing that bisexual plants are more
variable than unisexual, it will be interesting. I shall be very glad to
read the discussion which you are preparing. I admit as fully as any
one can do that cross-impregnation is the great check to endless
variability; but I am not sure that I understand your view. I do not
believe that the structure of Primula has any necessary relation to
a tendency to a dioecious structure, but seeing the difference in the
fertility of the two forms, I felt bound unwillingly to admit that they
might be a step towards dioeciousness; I allude to this subject in
my Linum paper. (638/3. "Linn. Soc. Journal," 1863.) Thanks for your
answers to my other queries. I forgot to say that I was at Kew the other
day, and I find that they can give me capsules of several Vandeae.

LETTER 639. TO J. SCOTT. Down, March 24th [1863].

Your letter, as every one you have written, has greatly interested me.
If you can show that certain individual Passifloras, under certain known
or unknown conditions of life, have stigmas capable of fertilisation
by pollen from another species, or from another individual of its own
species, yet not by its own individual pollen (its own individual pollen
being proved to be good by its action on some other species), you will
add a case of great interest to me; and which in my opinion would be
quite worth your publication. (639/1. Cases nearly similar to those
observed by Scott were recorded by Gartner and Kolreuter, but in these
instances only certain individuals were self-impotent. In "Animals and
Plants," Edition II., Volume II., page 114, where the phenomenon is
fully discussed, Scott's observations ("Trans. Bot. Soc. Edin." 1863)
are given as the earliest, except for one case recorded by Lecoq
("Fecondation," 1862). Interesting work was afterwards done by
Hildebrand and Fritz Muller, as illustrated in many of the letters
addressed to the latter.) I always imagined that such recorded cases
must be due to unnatural conditions of life; and think I said so in the
"Origin." (639/2. See "Origin of Species," Edition I., page 251, for
Herbert's observations on self-impotence in