Objection 1. It would seem that consanguinity is unsuitably defined by some as follows: "Consanguinity is the tie contracted between persons descending from the same common ancestor by carnal procreation." For all men descend from the same common ancestor, namely Adam, by carnal procreation. Therefore if the above definition of consanguinity is right, all men would be related by consanguinity: which is false.
Objection 2. Further, a tie is only between things in accord with one another, since a tie unites. Now there is not greater accordance between persons descended from a common ancestor than there is between other men, since they accord in species but differ in number, just as other men do. Therefore consanguinity is not a tie.
Objection 3. Further, carnal procreation, according to the Philosopher (De Gener. Anim. ii, 19), is effected from the surplus food [Cf. I, 119, 2]. Now this surplus has more in common with that which is eaten, since it agrees with it in substance, than with him who eats. Since then no tie of consanguinity arises between the person born of semen and that which he eats, neither will there be any tie of kindred between him and the person of whom he is born by carnal procreation.
Objection 4. Further, Laban said to Jacob (Genesis 29:14): "Thou art my bone and my flesh," on account of the relationship between them. Therefore such a kinship should be called flesh-relationship rather than blood-relationship [consanguinitas].
Objection 5. Further, carnal procreation is common to men and animals. But no tie of consanguinity is contracted among animals from carnal procreation. Therefore neither is there among men.
I answer that, According to the Philosopher (Ethic. iii, 11, 12) "all friendship is based on some kind of fellowship." And since friendship is a knot or union, it follows that the fellowship which is the cause of friendship is called "a tie." Wherefore in respect of any kind of a fellowship certain persons are denominated as though they were tied together: thus we speak of fellow-citizens who are connected by a common political life, of fellow-soldiers who are connected by the common business of soldiering, and in the same way those who are connected by the fellowship of nature are said to be tied by blood [consanguinei]. Hence in the above definition "tie" is included as being the genus of consanguinity; the "persons descending from the same common ancestor," who are thus tied together are the subject of this tie. while "carnal procreation" is mentioned as being its origin.
Reply to Objection 1. An active force is not received into an instrument in the same degree of perfection as it has in the principal agent. And since every moved mover is an instrument, it follows that the power of the first mover in a particular genus when drawn out through many mediate movers fails at length, and reaches something that is moved and not a mover. But the power of a begetter moves not only as to that which belongs to the species, but also as to that which belongs to the individual, by reason of which the child is like the parent even in accidentals and not only in the specific nature. And yet this individual power of the father is not so perfect in the son as it was in the father, and still less so in the grandson, and thus it goes on failing: so that at length it ceases and can go no further. Since then consanguinity results from this power being communicated to many through being conveyed to them from one person by procreation, it destroys itself by little and little, as Isidore says (Etym. ix). Consequently in defining consanguinity we must not take a remote common ancestor but the nearest, whose power still remains in those who are descended from him.
Reply to Objection 2. It is clear from what has been said that blood relations agree not only in the specific nature but also in that power peculiar to the individual which is conveyed from one to many: the result being that sometimes the child is not only like his father, but also his grandfather or his remote ancestors (De Gener. Anim. iv, 3).
Reply to Objection 3. Likeness depends more on form whereby a thing is actually, than on matter whereby a thing is potentially: for instance, charcoal has more in common with fire than with the tree from which the wood was cut. In like manner food already transformed by the nutritive power into the substance of the person fed has more in common with the subject nourished than with that from which the nourishment was taken. The argument however would hold according to the opinion of those who asserted that the whole nature of a thing is from its matter and that all forms are accidents: which is false.
Reply to Objection 4. It is the blood that is proximately changed into the semen, as proved in De Gener. Anim. i, 18. Hence the tie contracted by carnal procreation is more fittingly called blood-relationship than flesh-relationship. That sometimes one relation is called the flesh of another, is because the blood which is transformed into the man's seed or into the menstrual fluid is potentially flesh and bone.
Reply to Objection 5. Some say that the reason why the tie of consanguinity is contracted among men through carnal procreation, and not among other animals, is because whatever belongs to the truth of human nature in all men was in our first parent: which does not apply to other animals. But according to this, matrimonial consanguinity would never come to an end. However the above theory was disproved in the Second Book (Sent. ii, D, 30: I, 119, 1). Wherefore we must reply that the reason for this is that animals are not united together in the union of friendship through the begetting of many from one proximate parent, as is the case with men, as stated above.
Source: Summa Theologica (New Advent)