heir' are taken to mean 'if
he shall neither be heir himself nor cause another to be heir'; but when
applied to a person whom the testator supposes to be independent, they
mean 'if he shall not acquire the inheritance either for himself, or for
that person to whose power he shall subsequently become subject,' and
this was decided by Tiberius Caesar in the case of his slave Parthenius.

TITLE XVI. OF PUPILLARY SUBSTITUTION

To children below the age of puberty and in the power of the testator,
not only can such a substitute as we have described be appointed, that
is, one who shall take on their failing to inherit, but also one who
shall be their heir if, after inheriting, they die within the age of
puberty; and this may be done in the following terms, 'Be my son Titius
my heir; and if he does not become my heir, or, after becoming my heir,
die before becoming his own master (that is, before reaching puberty),
then be Seius my heir.' In which case, if the son fails to inherit,
the substitute is the heir of the testator; but if the son, after
inheriting, dies within the age of puberty, he is the heir of the son.
For it is a rule of customary law, that when our children are too young
to make wills for themselves, their parents may make them for them.

1 The reason of this rule has induced us to assert in our Code a
constitution, providing that if a testator has children, grandchildren,
or greatgrandchildren who are lunatics or idiots, he may, after the
analogy of pupillary substitution, substitute certain definite persons
to them, whatever their sex or the nearness of their relationship to
him, and even though they have reached the age of puberty; provided
always that on their recovering their faculties such substitution shall
at once become void, exactly as pupillary substitution proper ceases to
have any operation after the pupil has reached puberty.

2 Thus, in pupillary substitution effected in the form described, there
are, so to speak, two wills, the father's and the son's, just as if the
son had personally instituted an heir to himself; or rather, there is
one will dealing with two distinct matters, that is, with two distinct
inheritances.

3 If a testator be apprehensive that, after his own death, his son,
while still a pupil, may be exposed to the danger of foul play, because
another person is openly substituted to him, he ought to make the
ordinary substitution openly, and in the earlier part of the tes