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Whether the cause of similarity or dissimilarity among the members of various animals occurs as a result of the complexion of mixables or on account of the soul.
Certain members of animals," etc. With respect to this second book, one asks whether the cause of similarity or dissimilarity among the members of various animals occurs as a result of the complexion of mixables or on account of the soul.
1. It seems that this is not on account of the soul. This is because a cause of diverse things in a species produces a diverse effect in the species. But the souls of different animals differ in their species. Since, then, in the species their members are similar, that similarity cannot occur on account of the soul.
2. Likewise, the likeness between the heads of Sortis and Plato is greater than the likeness between Sortis's head and his hand. If, then, the similarity occurred on account of the soul, the head and the hand would agree more in appearance than would the head with the head.
3. Likewise, before the organization in the seed, the soul exists only virtually [virtute]. Therefore the similarity or dissimilarity of members after the organization cannot be on account of the soul.
On the contrary, the form of a substantial whole is the form of any given part. Therefore, just as diversity in the whole occurs from the form of the whole in the species, so too does diversity in the parts.
One should say that similarity and dissimilarity among the members is effectively on account of the soul of the one generating, and formally from the form of the mixed. That a fetus has just this sort of head or just this sort of foot occurs passively on account of the seed and actively on account of the power of the soul of the one generating. For a certain power of the one generating is split off from it at the same time as the seed is, and this acts in the sperm by its power and acts to produce one like the one generating. Therefore, because the seed derives from the superfluity of the nourishment of the one generating and is receptive to the form of the one generating, and because the power in the seed is able to produce a soul like itself, the power therefore organizes the semen itself so that the fetus's parts become like those of the one generating. But the seed itself is heterogeneous in power, although it is homogeneous in act. I call something heterogeneous that has one of its parts converted into flesh specifically and another is converted into bone and another is converted into the principal members. This is because members are formed variously on account of the variation existing in the mixing together of the parts of the seed, just as will be shown below. Thus, similarity among the members of different individuals in the same species occurs materially from the proportion of the mixables, and formally on account of the form of the species, and effectively from the agent, which is the power in the sperm.
1. One must reply to the first argument that if similarity and dissimilarity in the members occurred solely on account of the soul, then a different soul would produce dissimilar members in the species. Now, however, a proportion of the mixables is required for the members, and for that reason, if the proportion is the same in the one being acted upon, nothing prevents a likeness in species from being produced from diverse generating entities.
2. One should respond in the same way to the second argument, because the power in the seed is the active agent of different things in different ones, because it acts in the power of the one generating. Therefore, from the same material, or from one part of the material, it forms a hand, and from another part it forms a foot or an eye, but does so from a different proportion of the mixables.
3. To the third argument one should say that while the soul exists virtually [in virtute], the members are not organized. Rather, that power which acts to introduce the soul organizes them itself. And organization occurs on account of the powers of the soul to be introduced, because different organs are necessary for different tasks. This argument, however, clearly proves that for similarity in species a like proportion of the mixables is necessary. And this we concede.
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