Now there is one property that animals are found to have in common with plants. For some plants are generated from the seed of plants, whilst other plants are self-generated through the formation of some elemental principle similar to a seed; and of these latter plants some derive their nutriment from the ground, whilst others grow inside other plants, as is mentioned, by the way, in my treatise on Botany. So with animals, some spring from parent animals according to their kind, whilst others grow spontaneously and not from kindred stock; and of these instances of spontaneous generation some come from putrefying earth or vegetable matter, as is the case with a number of insects, while others are spontaneously generated in the inside of animals out of the secretions of their several organs
This is the same as the doctrine of St. Thomas Aquinas:
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Anima sensitiva educitur de potentia materiae in brutis. In nobis vero non, sed est per creationem, cuм eius essentia sit essentia animae rationalis, quae est per creationem" (Quodl, 11, 5, ad 1).
I do not doubt that both theories have their difficulties. The individuation of twins, for example, speaking of the theory of direct infusion, as well as the fact that about 50% of pregnancies fail, usually unnoticed, in the very first stage.
If this were not a problem, the Congregation for Studies should have formulated the XV. thomistic thesis differently:
XV. Contra, per se subsistit anima humana, quae, cuм subiecto sufficienter disposito potest infundi, a Deo creatur, et sua natura incorruptibilis est atque immortalis., relying of course on St. Thomas in his definition of soul generally:
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Anima est actus primus physici corporis organici potentia vitam habentis." (CG, 2, 61)
Also, as far as I can see it, the thomist moral theologians do not differentiate the gravity of an abortion according to the state of the fetus, even though the ascribe to the theory of St. Thomas.