on.

Now it would appear, that no secretion or excretion of fluid is attended
with so much agreeable sensation, as that of the semen; and it would thence
follow, that the glands, which perform this secretion, are more likely to
be much affected by their catenations with pleasurable sensations. This
circumstance is certain, that much more of this fluid is produced in a
given time, when the object of its exclusion is agreeable to the mind.

2. A forceable argument, which shews the necessity of pleasurable sensation
to copulation, is, that the act cannot be performed without it; it is
easily interrupted by the pain of fear or bashfulness; and no efforts of
volition or of irritation can effect this process, except such as induce
pleasurable ideas or sensations. See Sect. XXXIII. 1. 1.

A curious analogical circumstance attending hermaphrodite insects, as
snails and worms, still further illustrates this theory; if the snail or
worm could have impregnated itself, there might have been a saving of a
large male apparatus; but as this is not so ordered by nature, but each
snail and worm reciprocally receives and gives impregnation, it appears,
that a pleasurable excitation seems also to have been required.

This wonderful circumstance of many insects being hermaphrodites, and at
the same time not having power to impregnate themselves, is attended to by
Dr. Lister, in his Exercitationes Anatom. de Limacibus, p. 145; who,
amongst many other final causes, which he adduces to account for it, adds,
ut tam tristibus et frigidis animalibus majori cum voluptate perficiatur
venus.

There is, however, another final cause, to which this circumstance may be
imputed: it was observed above, that vegetable buds and bulbs, which are
produced without a mother, are always exact resemblances of their parent;
as appears in grafting fruit-trees, and in the flower-buds of the dioiceous
plants, which are always of the same sex on the same tree; hence those
hermaphrodite insects, if they could have produced young without a mother,
would not have been, capable of that change or improvement, which is seen
in all other animals, and in those vegetables, which are procreated by the
male embryon received and nourished by the female. And it is hence
probable, that if vegetables could only have been produced by buds and
bulbs, and not by sexual generation, that there would not at this time have
existed one thousandth part of their present number of species;