ly Obj. 4: Every virtue of a passible body is weakened by
continuous action, because such agents are also patient. Therefore
the transforming virtue is strong at first so as to be able to
transform not only enough for the renewal of what is lost, but also
for growth. Later on it can only transform enough for the renewal of
what is lost, and then growth ceases. At last it cannot even do this;
and then begins decline. In fine, when this virtue fails altogether,
the animal dies. Thus the virtue of wine that transforms the water
added to it, is weakened by further additions of water, so as to
become at length watery, as the Philosopher says by way of example
(De Gener. i, 5).

Reply Obj. 5: As the Philosopher says (De Gener. i, 5), when a
certain matter is directly transformed into fire, then fire is said
to be generated anew: but when matter is transformed into a fire
already existing, then fire is said to be fed. Wherefore if the
entire matter together loses the form of fire, and another matter
transformed into fire, there will be another distinct fire. But if,
while one piece of wood is burning, other wood is laid on, and so on
until the first piece is entirely consumed, the same identical fire
will remain all the time: because that which is added passes into
what pre-existed. It is the same with living bodies, in which by
means of nourishment that is renewed which was consumed by natural
heat.
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SECOND ARTICLE [I, Q. 119, Art. 2]

Whether the Semen Is Produced from Surplus Food?

Objection 1: It would seem that the semen is not produced from the
surplus food, but from the substance of the begetter. For Damascene
says (De Fide Orth. i, 8) that "generation is a work of nature,
producing, from the substance of the begetter, that which is
begotten." But that which is generated is produced from the semen.
Therefore the semen is produced from the substance of the begetter.

Obj. 2: Further, the son is like his father, in respect of that which
he receives from him. But if the semen from which something is
generated, is produced from the surplus food, a man would receive
nothing from his grandfather and his ancestors in whom the food never
existed. Therefore a man would not be more like to his grandfather or
ancestors, than to any other men.

Obj. 3: Further, the food of the generator is sometimes the flesh of
cows, pigs and suchlike. If therefore, the semen were produced from
surplus food, the m