s.
These cases are probably not due to similar conditions acting on similar
constitutions, but to a partial reversion in colour to the common
progenitor of these two species, as well as of the other species of the
genus. We shall hereafter have to return to this subject, and discuss it
more fully.

* * * * *

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CHAPTER III.

PIGS--CATTLE--SHEEP--GOATS.

PIGS BELONG TO TWO DISTINCT TYPES, SUS SCROFA AND
INDICA--TORF-SCHWEIN--JAPAN PIG--FERTILITY OF CROSSED PIGS--CHANGES IN
THE SKULL OF THE HIGHLY CULTIVATED RACES--CONVERGENCE OF
CHARACTER--GESTATION--SOLID-HOOFED SWINE--CURIOUS APPENDAGES TO THE
JAWS--DECREASE IN SIZE OF THE TUSKS--YOUNG PIGS LONGITUDINALLY
STRIPED--FERAL PIGS--CROSSED BREEDS.

CATTLE.--ZEBU A DISTINCT SPECIES--EUROPEAN CATTLE PROBABLY DESCENDED
FROM THREE WILD FORMS--ALL THE RACES NOW FERTILE TOGETHER--BRITISH PARK
CATTLE--ON THE COLOUR OF THE ABORIGINAL SPECIES--CONSTITUTIONAL
DIFFERENCES--SOUTH AFRICAN RACES--SOUTH AMERICAN RACES--NIATA
CATTLE--ORIGIN OF THE VARIOUS RACES OF CATTLE.

SHEEP.--REMARKABLE RACES OF--VARIATIONS ATTACHED TO THE MALE
SEX--ADAPTATIONS TO VARIOUS CONDITIONS--GESTATION OF--CHANGES IN THE
WOOL--SEMI-MONSTROUS BREEDS.

GOATS.--REMARKABLE VARIATIONS OF.

The breeds of the pig have recently been more closely studied, though much
still remains to be done, than those of almost any other domesticated
animal. This has been effected by Hermann von Nathusius in two admirable
works, especially in the later one on the Skulls of the several races, and
by Ruetimeyer in his celebrated Fauna of the ancient Swiss
lake-dwellings.[144] Nathusius has shown that all the known breeds may be
divided in two great groups: one resembling in all important respects and
no doubt descended from the common wild boar; so that this may be called
the _Sus scrofa_ group. The other group differs in several important and
constant osteological characters; its wild parent-form is unknown; the name
given to it by Nathusius, according to the law of priority, is _Sus Indica_
of Pallas. This name must now be followed, though an unfortunate one, as
the wild aboriginal does not inhabit India, and the best-known domesticated
breeds have been imported from Siam and China.

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Firstly, the _Sus scrofa_ breeds, or those resembling the common wild boar.
These still exist, according to Nathusius (Schweineschaedel,