Genesis 6:19

And of every living thing of all flesh, two of every sort
shalt thou bring into the ark
That is, of fowls, cattle, and creeping things, as after explained; and two of each sort at least were to be brought, as Jarchi observes, and not fewer; though of the clean sort there were to be more, even seven, as after directed; and these were to be brought, that they might preserve their species, as it follows: to keep [them] alive with thee;
to be fed and nourished by him in the ark, while others perished by the flood, that so they might propagate their own species, and be continued, for which reason it is further ordered: they shall be male and female;
not any two, but one male and one female, for the end before mentioned.