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Whether flyers naturally have to generate by laying eggs.

It is necessary that in addition to what we said, . . ." etc. This sixth book is mainly concerned with egg-laying. And one asks first whether flyers naturally have to generate by laying eggs.

1. And it seems not. For subtle and light things are generated more quickly, and this is why males are formed in the uterus more quickly than are females. But flyers are subtler and lighter than walkers. Since, then, walkers generate without an egg acting as a medium, and since live young come forth from a womb, how much more so should this be the case in flyers?

2. Moreover, coitus is the path to generation. Now, where coitus occurs more quickly, generation occurs more quickly. But flyers have intercourse most quickly, and therefore, etc.

3. Moreover, if flyers were generated from eggs, this would occur on account of their complexion. The conclusion is false, and therefore the premise [is false]. The conclusion is evident, because the first qualities are the causes of all posterior acts; for odors and colors and tastes are caused on account of these. The falsehood of the conclusion is evident because, if complexion were the cause of egg-laying, all egg-layers would have the same complexion. But this is not the case. For fish are cold and moist, and flyers are warm and dry, but they lay eggs notwithstanding.

The opposite is evident to the senses, that flyers are generated by laying eggs.

One must say that flyers are generated by egg-laying, and the same is true for certain animals living in water, like fish, and for certain ones living on land, like serpents and lizards. There are many reasons for this. For these animals are swift of movement, and for that reason they have a narrow body and a narrow womb, or nature provides for such a narrowing of the body and the womb, so much so that the animal cannot be formed in the womb but the animal is formed with something serving to mediate the process, namely, an egg.

Another reason lies in the great number of fetuses, because these animals produce a lot of sperm, and all of the semen cannot be absorbed at the same time in the womb on account of its narrowness; rather one part of the semen is expelled before another is formed, and for that reason they lay eggs in succession and lay them down underneath themselves. Scantiness of nourishment also contributes to this. Whence it can be said briefly that such animals generate by laying eggs on account of the weakness of their power. For there is a great number of fetuses in them, but they do not possess potent power for retaining the semen wherever the fetus is formed. As a result, it does not wait to be formed in the uterus, but rather is first expelled into the egg and is formed externally, and this is why nature provides a web and a shell for the egg, so that the fetus will be preserved until its formation is complete.

1. On to the arguments. To the first, one should say that although flyers are subtle and light in comparison to walkers, their powers are nevertheless weaker, and because of the weakness of their powers they are generated through a medium, namely, through the egg and its shell.

2. To the second argument one should say that it is just this way with regard to generation and to coition, because the opening of the pathways is sufficient for coition. This is especially found to be the case in flyers since they are warm and dry, and this is why they have intercourse very quickly. But generation results immediately from the strength of the [generative] power, and this is deficient in fish and serpents or reptiles owing to their lack of heat and is deficient in flyers because of their lack of moisture, because life demands proportionate heat and moisture.

3. To the third argument one must respond that complexion is the cause of egg-laying in flyers. Yet this complexion is not the precise cause, but the complexion regulated by the power of the soul. And this is why it is not necessary that all egg-layers share in the same specific complexion. But something is found to be shared by all things that lay eggs, for example, swiftness of motion, narrowness of the womb, a great number of fetuses, a great quantity of sperm, and a weakness in the retentive and formative power.

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