There is no more inherent improbability in each domestic pig, during a
thousand generations, retaining the capacity and tendency to develop great
tusks under fitting conditions, than in the young calf having retained, for an
indefinite number of generations rudimentary incisor teeth, which never
protrude through the gums.

I shall give at the end of the next chapter a summary of the three preceding
chapters; but as isolated and striking cases of reversion have here been
chiefly insisted on, I wish to guard the reader against supposing that
reversion is due to some rare or accidental combination of circumstances. When
a character, lost during hundreds of generations, suddenly reappears, no doubt
some such combination must occur; but reversions, to the immediately preceding
generations may be constantly observed, at least, in the offspring of most
unions. This has been universally recognised in the case of hybrids and
mongrels, but it has been recognised simply from the difference between the
united forms rendering the resemblance of the offspring to their grandparents
or more remote progenitors of easy detection. Reversion is likewise almost
invariably the rule, as Mr. Sedgwick has shown, with certain diseases. Hence
we must conclude that a tendency to this peculiar form of transmission is an
integral part of the general law of inheritance.

MONSTROSITIES.

A large number of monstrous growths and of lesser anomalies are admitted by
every one to be due to an arrest of development, that is, to the persistence
of an embryonic condition. But many monstrosities cannot be thus explained;
for parts of which no trace can be detected in the embryo, but which occur in
other members of the same class of animals occasionally appear, and these may
probably with truth be attributed to reversion. As, however, I have treated
this subject as fully as I could in my 'Descent of Man' (ch. 1 2nd edition), I