(25) Accordingly, in the larger definitions the number of the species that are discovered are in the ten Categories; and in the least, the principal points of the nearest species being taken, mark the essence and nature of the thing. But the least consists of three, the genus and two essentially necessary species.

(20) It is best, then, to divide the genus into two, if not into three species.

(18) Whence both these divisions are to be rejected, and only the division of the genus into species is approved, by which both the identity that is in the genus is characterized, and the diversity which subsists in the specific differences.

(9) Accordingly we must first take the genus, in which are the points that are nearest those above; and after this the next difference. And the succession of differences, when cut and divided, completes the "What it is." There is no necessity for expressing all the differences of each thing, but those which form the species.

(12) For, after dividing "the animal" into mortal and immortal, then into terrestrial and aquatic; and the terrestrial again into those who fly and those who walk; and so dividing the species which is nearest to what is sought, which also contains what is sought, we arrive by division at the simplest species, which contains nothing else, but what is sought alone.

(44) Next in the ascending scale come the Vertebrates, so called by reason of the presence in them of a vertebra or spinal column, or "backbone," and an internal skeleton as contrasted with the external skeleton of the lower forms of life. At the lowest end of the scale of the vertebrates are found the great family of Fishes, with high and low species. Then come the Reptiles, with its species of snakes, lizards, turtles, crocodiles, etc. There are many "connecting links" between the family of Fishes and that of the Reptiles; and also many between the family of Reptiles, and the family of Birds which is next highest in the scale. Among the birds, particularly in the Crow family, we find examples of a high degree of intelligence.