Aristotle has just shown that the species of a primary substance is more of a substance than the genus. He continues the Categories by showing that some substances are equally substance.
But of the species themselves—those which are not genera—one is no more a substance than another: it is no more apt to say of the individual man that he is a man than to say of the individual horse that it is a horse. And similarly of the primary substances one is no more a substance than another: the individual man is no more a substance than the individual ox.
Consider species that are not themselves the genus of anything. Every single species of this kind is just as much of a substance as any other species of this kind. Primary substances are all equally substances as well. No primary substance is more or less of a substance than any other primary substance. This means that men and horses are both substances and neither is more of a substance than the other. Similarly, no human being is more of a substance than any other human being.
In later terminology, species that are not themselves the genus of anything are called infimic species. This is an important piece of vocabulary because this specific kind of species is frequently referred to by Aristotle and those who came after him.
Next, Aristotle shows that secondary substances are limited to the genus and species of primary substances.