Whether an animal having parts of animals different in species can be generated from two animals that are different in species.
Next one inquires whether an animal having parts of animals different in species can be generated from two animals that are different in species.
1. It seems not. For according to Porphyry, species are un-mixed. This would not be the case, however, if a generation of this type were possible.
2. In addition, the womb of one species is not proportioned to the sperm of another. Therefore, if they should have intercourse, they would not produce a fetus.
The Philosopher implies the opposite.
One must respond that the production of such animals having the parts of animals different in species does not always result from animals that are different in species, since the mating seasons of different animals are different, as the Philosopher suggests. For one animal is impregnated at one time when another is not. Thus an animal having different parts.
1. On to the arguments. To the first, one must respond that species are unmixed in such a way that one is not in another, as some propose, and yet, a third species can come to be from two different species, as something pale or dark can be produced from black and white. Thus the premise is correct. The definition of a species arises from its form, and two forms are not conjoined in such an animal, and therefore neither are species. But the matter derived from each can be mixed, and the form is introduced according to the disposition of the matter and the nature of the agent power. Nevertheless, the production of such an animal departs greatly from the order of nature, and so it happens rarely.
2. To the second argument one must reply that if the womb and the sperm are not proportionate in their own properties, seasons, although his language sometimes seems to allow the reader to understand "gestation period" [spatium impregnationis in DA, e.g., 22.214.171.124 (SZ 2:1324)] they can nevertheless be proportionate in their common properties. And this is why, etc.
Whether milk is necessary for the nourishment of the fetus.
Next one inquires whether milk is necessary for the nourishment of the fetus after birth.
1. It seems not. After birth the fetus is complete, and therefore it needs a complete nutriment. But milk is not such a thing, whereas something born from the ground is, and therefore, etc.
2. In addition, the offspring (that is, the fetuses) of oviparous animals are not more complete than the offspring of those that produce milk. But the offspring of oviparous animals do not require milk for their nourishment after birth. Therefore, neither do the fetuses of walkers.
3. In addition, nourishment preserves life. Therefore, it especially should be hot and moist. But according to physicians, milk is cold, which is apparent in things produced from milk, like cheese and whey.
The Philosopher says the opposite.
One must reply that nature proceeds in an ordered manner when it operates. Now, however, the fetus is nourished in the uterus by menstrual blood, and when it is born the fetus is more complete outside the uterus than it was earlier. Therefore, it requires a more complete nourishment. But there are parts of the fetus that are not so solid or complete as to be able to digest or convert earthborn materials perfectly, and this is why, immediately after birth, nature gives a nourishment that is more digestible than menses and more refined than earthborn materials. But milk is like this. An indication of its digestion is its whiteness, because at first, during digestion, heat proceeds by whitening, and then later by darkening. And this is why milk appears in parts of the body that are hot, like the breasts, which are located on the chest in the human and on the belly in other animals, where heat is more abundant, and this is why hairs on the belly are whiter.
1. On to the arguments. To the first, one must reply that immediately after birth the fetus is not so complete or constructed that it can digest earthborn things adequately, but because it is nourished by the menstrual blood before birth, it is therefore nourished afterward by milk, which is somewhat removed from menstrual blood, because it is more digestible.
2. To the second argument one must respond that those born from eggs are nourished by the yolk since oviparous animals do not give birth to offspring inside the uterus but outside of it, and this is why they are not nourished by milk.
3. To the third argument one must reply that milk is naturally cold, because the earthy and watery abound in milk. But it nevertheless participates in heat with regard to having a more complete digestion and with regard to its container. It is unnecessary for the nutriment to be hot and moist at the beginning [of digestion] since at the beginning, the nutriment has to be dissimilar to that which it nourishes, but has to be similar to it at the end, as is said in the second book of On the Soul.
-  Porphyry, Isagoge, trans. Boethius, (Brandt, 1906), 334, 11.
-  A. uses the term tempus impregnationis and is apparently speaking of matly parts of these animals, is not generated from the joining of these two, but rather an animal like this is generated from a defect in the agent. For if the proper power is not present in the male's semen, and the power of the species does not dominate, but only the power of the genus dominates, then an animal is produced, and it is not like the male in species. Likewise, to the extent that the power either variously prevails or is overcome, to that extent the fetus will vary. If, however, two animals have suitable times for impregnation, then it is possible that the womb of one will share properties common to the sperm of the other, even though they may differ in their own individual properties. And because they are proportionate in common properties, an animal is therefore produced, and yet it will resemble neither the father nor mother, as is evident in the generation of a mule. If, however, the power in the sperm of the male prevails over some parts of the matter and is overcome by others, then, to the extent that it prevails, it causes the fetus to resemble the father, and to the extent that it is overcome, the mother. And in this way it is possible for such an animal, diversified in its parts, to be generated from different ones.
-  Perhaps, "in principle."
-  The logic seems to be that if our food were similar to ourselves when we ate it, then, logically, our bodies would routinely digest themselves. Rather, the act of digestion must convert something unlike us into something like us. Cf. Ar., De anima 2.4 (416a29f.).