he capacity for the production of an additional digit; and so in
other cases. Nevertheless there is no more inherent improbability in this
being the case than in a useless and rudimentary organ, or even in only a
tendency to the production of a rudimentary organ, being inherited during
millions of generations, as is well known to occur with a multitude of
organic beings. 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 may be constantly observed, at least to the immediately
preceding generations, 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. {57}

* * * * *

_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. If every horse or ass
had striped legs whilst young, the stripes which occasionally appear on
these animals when adult would have to be considered as due to the
anomalous retention of an early character, and not as due to reversion.
Now, the leg-stripes in the horse-genus, and some other characters in
analogous cases, are apt to occur during early youth and then to disappear;
thus the persistence of early characters and reversion are brought into
close connexion.

But many mon