ON PARTURITION. 541
Aristotle/ and already cited by me, which is, that all parts are made for a certain function, and if the function ceases to be required that they themselves disappear. The eye sees, the ear hears, the brain perceives, the stomach digests, not because such characters and structures (naturally) belong to these organs ; but they are endowed with such characters and structures to accomplish the functions appointed them by nature.
On grounds like these it would appear that the uterus holds the first place among the organs destined for generation ; for the testicles are made to produce semen, the semen is for the purposes of intercourse, and coition itself, or the emission of the semen, is instituted by nature that the uterus may be fecundated and generation result.
I have said before that an egg is, as it were, the fruit of an animal, and a kind of external uterus. Now, on the other hand, we may regard the uterus as an egg remaining within. For as trees are gay with leaves, flowers, and fruit at stated periods, and oviparous animals at one time conceive and produce eggs, at another become effete, so that neither the "place" or the part that contained them can be found, so have viviparous animals their spring and autumn allotted them. At the season of fecundation the genital organs, especially in the female, undergo great changes, so much so that in birds, the ovary, which at other times is scarcely visible, now becomes turgid; and the belly of the fish, near about the time of spawn- ing, far exceeds in bulk the rest of the body, owing to the enormous number of ova and the quantity of semen contained Avithin it. In very many viviparous animals the genital organs, that is, the uterus and spermatic vessels, are not always found presenting the same mode and course of action and structure ; but as they are capable or not of conception, so changes take place, and to such an extent that the organs can hardly be recognized as the same. In nature, just as there is nothing lacking, so is there nothing superfluous ; and thus it happens that the organs of generation wither away and are lost when there is no longer any use for them.
At the period of coitus in the hare and mole, the testicles
' De Gen. Anim. lib. i, cap. 5.
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