Yet he was not unaware of facts of form for which he could not quite find a place in his theory of organic form, facts of form which were not, at first sight at least, facts of function. Thus he was aware of certain facts of "correlation," which could not be explained off-hand as due to correlation of the functions of the parts. He knew, for instance, that all animals without front teeth in the upper jaw have cotyledons, while most that have front teeth on both jaws and no horns have no cotyledons (_De Gen._, ii. 7). Speaking generally, however, we find in Aristotle no purely morphological concepts. What then does morphology owe to Aristotle? It owes to him, _first_, a great mass of facts about the structure of animals; _second_, the first scientific classification of animals;[6] _third_, a clear enunciation of the fact of community of plan within each of the big groups; _fourth_, an attempt to explain certain instances of the correlation of parts; _fifth_, a pregnant distinction between homogeneous and heterogeneous parts; _sixth_, a generalisation on the succession of forms in development; and _seventh_, the first enunciation of the idea of the _Echelle des etres_. (1) What surprises the modern reader of the _Historia Animalium_ perhaps more than anything else is the extent and variety of Aristotle's knowledge of animals. He describes more than 500 kinds.[7] Not only does he know the ordinary beasts, birds, and fishes with which everyone is acquainted, but he knows a great deal about cuttlefish, snails and oysters, about crabs, crawfish (_Palinurus_), lobsters, shrimps, and hermit crabs, about sea-urchins and starfish, sea-anemones and sponges, about ascidians (which seem to have puzzled him not a little!). He has noticed even fish-lice and intestinal worms, both flat and round. Of the smaller land animals, he knows a great many insects and their larvae. The extent of his anatomical knowledge is equally surprising, and much of it is clearly the result of personal observation. No one can read his account of the internal anatomy of the chameleon (_Hist. Anim._, ii.), or his description of the structure of cuttlefish (_Hist. Anim._, iv), or that touch in the description of the hermit crab (_Hist. Anim._, iv.)--" Two large eyes ... not ... turned on one side like those of crabs, but straight forward"--without being convinced that Aristotle is speaking of what he has seen. Naturally he could not make much of the anatomy of small insects and snails, and, to tell the truth, he does not seem to have cared greatly about the minutiae of structure. He was too much of a Greek and an aristocrat to care about laborious detail.