seed,
and their offspring are more or less sterile; and these forms belong
to the same undoubted species, and differ from each other in no respect
except in their reproductive organs and functions.

Although the fertility of varieties when intercrossed, and of their
mongrel offspring, has been asserted by so many authors to be universal,
this cannot be considered as quite correct after the facts given on the
high authority of Gartner and Kolreuter. Most of the varieties which
have been experimented on have been produced under domestication; and as
domestication (I do not mean mere confinement) almost certainly tends
to eliminate that sterility which, judging from analogy, would have
affected the parent-species if intercrossed, we ought not to expect
that domestication would likewise induce sterility in their modified
descendants when crossed. This elimination of sterility apparently
follows from the same cause which allows our domestic animals to breed
freely under diversified circumstances; and this again apparently
follows from their having been gradually accustomed to frequent changes
in their conditions of life.

A double and parallel series of facts seems to throw much light on the
sterility of species, when first crossed, and of their hybrid offspring.
On the one side, there is good reason to believe that slight changes in
the conditions of life give vigour and fertility to all organic beings.
We know also that a cross between the distinct individuals of the same
variety, and between distinct varieties, increases the number of their
offspring, and certainly gives to them increased size and vigour. This
is chiefly owing to the forms which are crossed having been exposed
to somewhat different conditions of life; for I have ascertained by a
labourious series of experiments that if all the individuals of the same
variety be subjected during several generations to the same conditions,
the good derived from crossing is often much diminished or wholly
disappears. This is one side of the case. On the other side, we know
that species which have long been exposed to nearly uniform conditions,
when they are subjected under confinement to new and greatly changed
conditions, either perish, or if they survive, are rendered sterile,
though retaining perfect health. This does not occur, or only in a very
slight degree, with our domesticated productions, which have long been
exposed to fluctuating conditions. Hence when we find t