In this as in other fields, we shall be able to study our subject best if we begin at the beginning and consider things in the process of their growth.
First of all, there must necessarily be a union or pairing of those who cannot exist without one another.
Male and female must unite for the reproduction of the species-not from deliberate intention, but from the natural impulse, which exists in animals generally as it also exists in plants, to leave behind them something of the same nature as themselves.