to keep true than others; and M. Naudin, who has had such
immense experience in this group, informs me that he believes that
certain varieties intercross more readily than others of the same
species; but he has not proved the truth of this conclusion; the
frequent abortion of the pollen near Paris being one great difficulty.
Nevertheless, he has grown close 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. Koelreuter
minutely describes five varieties of the common tobacco,[236] which
were reciprocally crossed, and the offspring were intermediate in
character and as fertile as their parents: from this fact Koelreuter
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.[237] So that the sexual {109}
capacity of this one variety has certainly been in some degree
modified, so as to approach in nature that of _N. glutinosa_.[238]

These facts with respect to plants show that in some few cases certain
varieties have had their sexual powers so far modified, that they cross
together less readily and yield less seed than other varieties of the same
species. We shall presently see that the sexual functions of most animals
and plants are eminently liable to be affected by the conditions of life to
which they are exposed; and hereafter we shall briefly discuss the conjoint
bearing of this and other facts on the difference in fertility between
crossed varieties and crossed species.

_Domestication eliminates the tendency to Sterility which is general with
Species when crossed._

This hypothesis was first pr