the same natural species, uniformity of character, though largely
modified by natural selection and by the direct action of the surrounding
conditions.

_On the possibility of all organic beings occasionally intercrossing._--But
it may be asked, can free crossing occur with hermaphrodite animals and
plants? All the higher animals, and the few insects which have been
domesticated, have separated sexes, and must inevitably unite for each
birth. With respect to the crossing of hermaphrodites, the subject is too
large for the present volume, and will be more properly treated in a
succeeding work. In my 'Origin of Species,' however, I have given a short
abstract of the reasons which induce me to believe that all organic beings
occasionally cross, though perhaps in some cases only at long intervals of
time.[192] I will here just recall the fact that many plants, though
hermaphrodite in structure, are unisexual in function;--such as those
called by C. K. Sprengel _dichogamous_, in which the pollen and stigma of
the same flower are matured at different periods; or those called by me
_reciprocally dimorphic_, in which the flower's own pollen is not fitted to
fertilise its own stigma; or again, the many kinds in which curious
mechanical contrivances exist, effectually preventing self-fertilisation.
There are, however, many hermaphrodite plants which are not in any way
specially constructed to favour intercrossing, but which nevertheless
commingle almost as freely as animals with separated sexes. This is the
case with cabbages, radishes, and onions, as I know from {91} having
experimented on them: even the peasants of Liguria say that cabbages must
be prevented "from falling in love" with each other. In the orange tribe,
Gallesio[193] remarks that the amelioration of the various kinds is checked
by their continual and almost regular crossing. So it is with numerous
other plants.

Nevertheless some cultivated plants can be named which rarely intercross,
as the common pea, or which never intercross, as I have reason to believe
is the case with the sweet-pea (_Lathyrus odoratus_); yet the structure of
these flowers certainly favours an occasional cross. The varieties of the
tomato and aubergine (_Solanum_) and pimenta (_Pimenta vulgaris?_) are
said[194] never to cross, even when growing alongside each other. But it
should be observed that these are all exotic plants, and we do not know how
they would behave in their native country w