326 
DARWINISM 
CHAP. 
of the species or race, yet he clearly saw * that it was not 
always and necessarily advantageous. He says : “ The most 
important conclusion at which I have arrived is, that the mere 
act of intercrossing by itself does no good. The good 
depends on the individuals which are crossed differing slightly 
in constitution, owing to their progenitors having been sub¬ 
jected during several generations to slightly different con¬ 
ditions. This conclusion, as we shall hereafter see, is closely 
connected with various important physiological problems, such 
as the benefit derived from slight changes in the conditions of 
life.” 1 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. 2 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 inter¬ 
crossing. It appears probable, then, that it is not inter¬ 
breeding in itself that is hurtful, but interbreeding without 
1 Cross- and Self-Fertilisation, p. 27. 
2 Animals and Plants, vol. ii. p. 145. 
