Objection 1. It would seem that the species of goodness derived from the end is contained under the species of goodness derived from the object, as a species is contained under its genus; for instance, when a man commits a theft in order to give alms. For an action takes its species from its object, as stated above (2,6). But it is impossible for a thing to be contained under another species, if this species be not contained under the proper species of that thing; because the same thing cannot be contained in different species that are not subordinate to one another. Therefore the species which is taken from the end, is contained under the species which is taken from the object.
Objection 2. Further, the last difference always constitutes the most specific species. But the difference derived from the end seems to come after the difference derived from the object: because the end is something last. Therefore the species derived from the end, is contained under the species derived from the object, as its most specific species.
Objection 3. Further, the more formal a difference is compared to genus, as form to matter. But the species derived from the end, is more formal than that which is derived from the object, as stated above (Article 6). Therefore the species derived from the end is contained under the species derived from the object, as the most specific species is contained under the subaltern genus.
On the contrary, Each genus has its determinate differences. But an action of one same species on the part of its object, can be ordained to an infinite number of ends: for instance, theft can be ordained to an infinite number of good and bad ends. Therefore the species derived from the end is not contained under the species derived from the object, as under its genus.
I answer that, The object of the external act can stand in a twofold relation to the end of the will: first, as being of itself ordained thereto; thus to fight well is of itself ordained to victory; secondly, as being ordained thereto accidentally; thus to take what belongs to another is ordained accidentally to the giving of alms. Now the differences that divide a genus, and constitute the species of that genus, must, as the Philosopher says (Metaph. vii, 12), divide that genus essentially: and if they divide it accidentally, the division is incorrect: as, if one were to say: "Animals are divided into rational and irrational; and the irrational into animals with wings, and animals without wings"; for "winged" and "wingless" are not essential determinations of the irrational being. But the following division would be correct: "Some animals have feet, some have no feet: and of those that have feet, some have two feet, some four, some many": because the latter division is an essential determination of the former. Accordingly when the object is not of itself ordained to the end, the specific difference derived from the object is not an essential determination of the species derived from the end, nor is the reverse the case. Wherefore one of these species is not under the other; but then the moral action is contained under two species that are disparate, as it were. Consequently we say that he that commits theft for the sake of adultery, is guilty of a twofold malice in one action. On the other hand, if the object be of itself ordained to the end, one of these differences is an essential determination of the other. Wherefore one of these species will be contained under the other.
It remains to be considered which of the two is contained under the other. In order to make this clear, we must first of all observe that the more particular the form is from which a difference is taken, the more specific is the difference. Secondly, that the more universal an agent is, the more universal a form does it cause. Thirdly, that the more remote an end is, the more universal the agent to which it corresponds; thus victory, which is the last end of the army, is the end intended by the commander in chief; while the right ordering of this or that regiment is the end intended by one of the lower officers. From all this it follows that the specific difference derived from the end, is more general; and that the difference derived from an object which of itself is ordained to that end, is a specific difference in relation to the former. For the will, the proper object of which is the end, is the universal mover in respect of all the powers of the soul, the proper objects of which are the objects of their particular acts.
Reply to Objection 1. One and the same thing, considered in its substance, cannot be in two species, one of which is not subordinate to the other. But in respect of those things which are superadded to the substance, one thing can be contained under different species. Thus one and the same fruit, as to its color, is contained under one species, i.e. a white thing: and, as to its perfume, under the species of sweet-smelling things. In like manner an action which, as to its substance, is in one natural species, considered in respect to the moral conditions that are added to it, can belong to two species, as stated above (1, 3, ad 3).
Reply to Objection 2. The end is last in execution; but first in the intention of the reason, in regard to which moral actions receive their species.
Reply to Objection 3. Difference is compared to genus as form to matter, inasmuch as it actualizes the genus. On the other hand, the genus is considered as more formal than the species, inasmuch as it is something more absolute and less contracted. Wherefore also the parts of a definition are reduced to the genus of formal cause, as is stated in Phys. ii, 3. And in this sense the genus is the formal cause of the species; and so much the more formal, as it is more universal.
Source: Summa Theologica (New Advent)