rable body of evidence,
that slight changes in the conditions of life are beneficial to all
living things. We see this acted on by farmers and gardeners in their
frequent exchanges of seed, tubers, etc., from one soil or climate to
another, and back again. During the convalescence of animals, we plainly
see that great benefit is derived from almost any change in the
habits of life. Again, both with plants and animals, there is abundant
evidence, that a cross between very distinct individuals of the same
species, that is between members of different strains or sub-breeds,
gives vigour and fertility to the offspring. I believe, indeed, from
the facts alluded to in our fourth chapter, that a certain amount of
crossing is indispensable even with hermaphrodites; and that close
interbreeding continued during several generations between the nearest
relations, especially if these be kept under the same conditions of
life, always induces weakness and sterility in the progeny.

Hence it seems that, on the one hand, slight changes in the conditions
of life benefit all organic beings, and on the other hand, that slight
crosses, that is crosses between the males and females of the same
species which have varied and become slightly different, give vigour and
fertility to the offspring. But we have seen that greater changes, or
changes of a particular nature, often render organic beings in some
degree sterile; and that greater crosses, that is crosses between males
and females which have become widely or specifically different, produce
hybrids which are generally sterile in some degree. I cannot persuade
myself that this parallelism is an accident or an illusion. Both series
of facts seem to be connected together by some common but unknown bond,
which is essentially related to the principle of life.

FERTILITY OF VARIETIES WHEN CROSSED, AND OF THEIR MONGREL OFFSPRING.

It may be urged, as a most forcible argument, that there must be some
essential distinction between species and varieties, and that there
must be some error in all the foregoing remarks, inasmuch as varieties,
however much they may differ from each other in external appearance,
cross with perfect facility, and yield perfectly fertile offspring. I
fully admit that this is almost invariably the case. But if we look to
varieties produced under nature, we are immediately involved in hopeless
difficulties; for if two hitherto reputed varieties be found in any
degree steril