together, during seven years, certain forms of Citrullus, which, as they could
be artificially crossed with perfect facility and produced fertile offspring,
are ranked as varieties; but these forms when not artificially crossed kept
true. Many other varieties, on the other hand, in the same group cross with
such facility, as M. Naudin repeatedly insists, that without being grown far
apart they cannot be kept in the least true.

Another case, though somewhat different, may be here given, as it is highly
remarkable, and is established on excellent evidence. Kolreuter minutely
describes five varieties of the common tobacco (16/25. 'Zweite Forts.' s. 53
namely Nicotiana major vulgaris; (2) perennis; (3) transylvanica; (4) a sub-
var. of the last; (5) major latifol. fl. alb.) which were reciprocally
crossed, and the offspring were intermediate in character and as fertile as
their parents: from this fact Kolreuter inferred that they are really
varieties; and no one, as far as I can discover, seems to have doubted that
such is the case. He also crossed reciprocally these five varieties with N.
glutinosa, and they yielded very sterile hybrids; but those raised from the
var. perennis, whether used as the father or mother plant, were not so sterile
as the hybrids from the four other varieties. (16/26. Kolreuter was so much
struck with this fact that he suspected that a little pollen of N. glutinosa
in one of his experiments might have accidentally got mingled with that of
var. perennis, and thus aided its fertilising power. But we now know
conclusively from Gartner ('Bastarderz.' s. 34, 43) that the pollen of two
species never acts CONJOINTLY on a third species; still less will the pollen
of a distinct species, mingled with a plant's own pollen, if the latter be
present in sufficient quantity, have any effect. The sole effect of mingling
two kinds of pollen is to produce in the same capsule seeds which yield
plants, some taking after the one and some after the other parent.) So that
the sexual capacity of this one variety has certainly been in some degree
modified, so as to approach in nature that of N. glutinosa. (16/27. Mr. Scott