each other,--all
varieties can be so grafted. Some allied animals are affected in a very
different manner by the same poison, but with varieties no such case until
recently was known, but now it has been proved that immunity from certain
poisons stands in some cases in correlation with the colour of the hair.
The period of gestation generally differs much with distinct species, but
with varieties until lately no such difference had been observed. The time
required for the germination of seeds differs in an analogous manner, and I
am not aware that any difference in this respect has as yet been detected
with varieties. Here we have various physiological differences, and no
doubt others could be added, between one species and another of the same
genus, which do not occur, or occur with extreme rarity, in the case of
varieties; and these differences are apparently wholly or in chief part
incidental on other constitutional differences, just in the same manner as
the sterility of crossed species is incidental on differences confined to
the sexual system. Why, then, should these latter differences, however
serviceable they may indirectly be in keeping the inhabitants of the same
country distinct, be thought of such paramount importance, in comparison
with other incidental and functional differences? No sufficient answer to
this question can be given. Hence the fact that the most distinct domestic
varieties are, with rare exceptions, perfectly fertile when crossed, and
produce fertile offspring, whilst closely allied species are, with rare
exceptions, more or less sterile, is not nearly so formidable an objection
as it appears at first to the theory of the common descent of allied
species.

* * * * *

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CHAPTER XX.

SELECTION BY MAN.

SELECTION A DIFFICULT ART--METHODICAL, UNCONSCIOUS, AND NATURAL
SELECTION--RESULTS OF METHODICAL SELECTION--CARE TAKEN IN
SELECTION--SELECTION WITH PLANTS--SELECTION CARRIED ON BY THE ANCIENTS,
AND BY SEMI-CIVILIZED PEOPLE--UNIMPORTANT CHARACTERS OFTEN ATTENDED
TO--UNCONSCIOUS SELECTION--AS CIRCUMSTANCES SLOWLY CHANGE, SO HAVE OUR
DOMESTICATED ANIMALS CHANGED THROUGH THE ACTION OF UNCONSCIOUS
SELECTION--INFLUENCE OF DIFFERENT BREEDERS ON THE SAME
SUB-VARIETY--PLANTS AS AFFECTED BY UNCONSCIOUS SELECTION--EFFECTS OF
SELECTION AS SHOWN BY THE GREAT AMOUNT OF DIFFERENCE IN THE PARTS MOST
VALUED BY MAN.

The power of