ht changes
in the conditions of life."[157] Mr. Darwin has also adduced much direct
evidence proving that slight changes in the conditions of life are
beneficial to both animals and plants, maintaining or restoring their
vigour and fertility in the same way as a favourable cross seems to
restore it.[158] It is, I believe, by a careful consideration of these
two classes of facts that we shall find the clue to the labyrinth in
which this subject has appeared to involve us.

_Supposed Evil Results of Close Interbreeding._

Just as we have seen that intercrossing is not necessarily good, we
shall be forced to admit that close interbreeding is not necessarily
bad. Our finest breeds of domestic animals have been thus produced, and
by a careful statistical inquiry Mr. George Darwin has shown that the
most constant and long-continued intermarriages among the British
aristocracy have produced no prejudicial results. The rabbits on Porto
Santo are all the produce of a single female; they have lived on the
same small island for 470 years, and they still abound there and appear
to be vigorous and healthy (see p. 161).

We have, however, on the other hand, overwhelming evidence that in many
cases, among our domestic animals and cultivated plants, close
interbreeding does produce bad results, and the apparent contradiction
may perhaps be explained on the same general principles, and under
similar limitations, as were found to be necessary in defining the value
of intercrossing. It appears probable, then, that it is not
interbreeding in itself that is hurtful, but interbreeding without
rigid selection or some change of conditions. Under nature, as in the
case of the Porto Santo rabbits, the rapid increase of these animals
would in a very few years stock the island with a full population, and
thereafter natural selection would act powerfully in the preservation
only of the healthiest and the most fertile, and under these conditions
no deterioration would occur. Among the aristocracy there has been a
constant selection of beauty, which is generally synonymous with health,
while any constitutional infertility has led to the extinction of the
family. With domestic animals the selection practised is usually neither
severe enough nor of the right kind. There is no natural struggle for
existence, but certain points of form and colour characteristic of the
breed are considered essential, and thus the most vigorous or the most
fertile are