w it is pretty clear that they must be liable to
crossing. Sweet-peas (583/1. In Lathyrus odoratus the absence of the
proper insect has been supposed to prevent crossing. See "Variation
under Domestication," Edition II., Volume II., page 68; but the
explanation there given for Pisum may probably apply to Lathyrus.),
bee-orchis, and perhaps hollyhocks are, at present, my greatest
difficulties; and I find I cannot experimentise by castrating
sweet-peas, without doing fatal injury. Formerly I felt most interest
on this point as one chief means of eliminating varieties; but I feel
interest now in other ways. One general fact [that] makes me believe in
my doctrine (583/2. The doctrine which has been epitomised as "Nature
abhors perpetual self-fertilisation," and is generally known as
Knight's Law or the Knight-Darwin Law, is discussed by Francis Darwin in
"Nature," 1898. References are there given to the chief passages in
the "Origin of Species," etc., bearing on the question. See Letter 19,
Volume I.), is that NO terrestrial animal in which semen is liquid is
hermaphrodite except with mutual copulation; in terrestrial plants in
which the semen is dry there are many hermaphrodites. Indeed, I do wish
I lived at Kew, or at least so that I could see you oftener. To return
again to subject of crossing: I have been inclined to speculate so far,
as to think (my!?) notion (I say MY notion, but I think others have put
forward nearly or quite similar ideas) perhaps explains the frequent
separation of the sexes in trees, which I think I have heard remarked
(and in looking over the mono- and dioecious Linnean classes in Persoon
seems true) are very apt to have sexes separated; for [in] a tree having
a vast number of flowers on the same individual, or at least the same
stock, each flower, if only hermaphrodite on the common plan, would
generally get its own pollen or only pollen from another flower on
same stock,--whereas if the sexes were separate there would be a better
chance of occasional pollen from another distinct stock. I have thought
of testing this in your New Zealand Flora, but I have no standard of
comparison, and I found myself bothered by bushes. I should propound
that some unknown causes had favoured development of trees and bushes
in New Zealand, and consequent on this there had been a development
of separation of sexes to prevent too much intermarriage. I do not, of
course, suppose the prevention of too much intermarriage