Objection 1. It seems that nothing can be generated from the sacramental species: because, whatever is generated, is generated out of some matter: for nothing is generated out of nothing, although by creation something is made out of nothing. But there is no matter underlying the sacramental species except that of Christ's body, and that body is incorruptible. Therefore it seems that nothing can be generated from the sacramental species.
Objection 2. Further, things which are not of the same genus cannot spring from one another: thus a line is not made of whiteness. But accident and substance differ generically. Therefore, since the sacramental species are accidents, it seems that no substance can be generated from them.
Objection 3. Further, if any corporeal substance be generated from them, such substance will not be without accident. Therefore, if any corporeal substance be generated from the sacramental species, then substance and accident would be generated from accident, namely, two things from one, which is impossible. Consequently, it is impossible for any corporeal substance to be generated out of the sacramental species.
On the contrary, The senses are witness that something is generated out of the sacramental species, either ashes, if they be burned, worms if they putrefy, or dust if they be crushed.
I answer that, Since "the corruption of one thing is the generation of another" (De Gener. i), something must be generated necessarily from the sacramental species if they be corrupted, as stated above (Article 4); for they are not corrupted in such a way that they disappear altogether, as if reduced to nothing; on the contrary, something sensible manifestly succeeds to them.
Nevertheless, it is difficult to see how anything can be generated from them. For it is quite evident that nothing is generated out of the body and blood of Christ which are truly there, because these are incorruptible. But if the substance, or even the matter, of the bread and wine were to remain in this sacrament, then, as some have maintained, it would be easy to account for this sensible object which succeeds to them. But that supposition is false, as was stated above (75, 2,4,8).
Hence it is that others have said that the things generated have not sprung from the sacramental species, but from the surrounding atmosphere. But this can be shown in many ways to be impossible. In the first place, because when a thing is generated from another, the latter at first appears changed and corrupted; whereas no alteration or corruption appeared previously in the adjacent atmosphere; hence the worms or ashes are not generated therefrom. Secondly, because the nature of the atmosphere is not such as to permit of such things being generated by such alterations. Thirdly, because it is possible for many consecrated hosts to be burned or putrefied; nor would it be possible for an earthen body, large enough to be generated from the atmosphere, unless a great and, in fact, exceedingly sensible condensation of the atmosphere took place. Fourthly, because the same thing can happen to the solid bodies surrounding them, such as iron or stone, which remain entire after the generation of the aforesaid things. Hence this opinion cannot stand, because it is opposed to what is manifest to our senses.
And therefore others have said that the substance of the bread and wine returns during the corruption of the species, and so from the returning substance of the bread and wine, ashes or worms or something of the kind are generated. But this explanation seems an impossible one. First of all, because if the substance of the bread and wine be converted into the body and blood of Christ, as was shown above (75, 2,4), the substance of the bread and wine cannot return, except the body and blood of Christ be again changed back into the substance of bread and wine, which is impossible: thus if air be turned into fire, the air cannot return without the fire being again changed into air. But if the substance of bread or wine be annihilated, it cannot return again, because what lapses into nothing does not return numerically the same. Unless perchance it be said that the said substance returns, because God creates anew another new substance to replace the first. Secondly, this seems to be impossible, because no time can be assigned when the substance of the bread returns. For, from what was said above (4; 76, 6, ad 3), it is evident that while the species of the bread and wine remain, there remain also the body and blood of Christ, which are not present together with the substance of the bread and wine in this sacrament, according to what was stated above (Question 75, Article 2). Hence the substance of the bread and wine cannot return while the sacramental species remain; nor, again, when these species pass away; because then the substance of the bread and wine would be without their proper accidents, which is impossible. Unless perchance it be said that in the last instant of the corruption of the species there returns (not, indeed, the substance of bread and wine, because it is in that very instant that they have the being of the substance generated from the species, but) the matter of the bread and wine; which, matter, properly speaking, would be more correctly described as created anew, than as returning. And in this sense the aforesaid position might be held.
However, since it does not seem reasonable to say that anything takes place miraculously in this sacrament, except in virtue of the consecration itself, which does not imply either creation or return of matter, it seems better to say that in the actual consecration it is miraculously bestowed on the dimensive quantity of the bread and wine to be the subject of subsequent forms. Now this is proper to matter; and therefore as a consequence everything which goes with matter is bestowed on dimensive quantity; and therefore everything which could be generated from the matter of bread or wine, if it were present, can be generated from the aforesaid dimensive quantity of the bread or wine, not, indeed, by a new miracle, but by virtue of the miracle which has already taken place.
Reply to Objection 1. Although no matter is there out of which a thing may be generated, nevertheless dimensive quantity supplies the place of matter, as stated above.
Reply to Objection 2. Those sacramental species are indeed accidents, yet they have the act and power of substance, as stated above (Article 3).
Reply to Objection 3. The dimensive quantity of the bread and wine retains its own nature, and receives miraculously the power and property of substance; and therefore it can pass to both, that is, into substance and dimension.
Source: Summa Theologica (New Advent)