The possession of one or more of the three types of soul, vegetative, sensitive, and rational, provides in itself a basis for an elementary form of arrangement of living things in an ascending scale. We have already seen that Aristotle certainly describes something resembling a ‘Scala Naturae’ and that such a scheme can easily be drawn up from passages in his works. It may, however, be doubted whether his phraseology is capable of extension so as to include a true _classification_ of animals in any modern sense. It is true that he repeatedly divides animals into classes, _Sanguineous_ and _Non-sanguineous_, _Oviparous_ and _Viviparous_, _Terrestrial_ and _Aquatic_, &c., but his divisions are for the most part simply dichotomic. He certainly defines a few groups of animals as the Lophura (_Equidae_), the Cete (_Cetacea_), and the Selache (_Elasmobranchiae_ together with the _Lophiidae_) in a way that fairly corresponds to similar groups in later systems. In most cases, however, his definitions are not exact enough for modern needs, for the same animal may fall into more than one of his classes and widely different animals into the same class. Thus he invents a category _Carcharodonta_ for animals with sharp interlocking teeth and includes in it carnivores, reptiles, and fish; again, the horse kind must be included both among his _Anepallacta_ or animals having flat crowned teeth as well as among the _Amphodonta_ or animals with front teeth in both jaws. Such words as these are really terms of _description_, not of classification in the modern biological sense of that word. There are, however, scattered through the biological works, certain terms which are applied to animal groups and organs and are defined in such a way as to suggest that they might ultimately have been developed for classificatory purposes. Thus his lowest group is the _species_. ‘The individuals comprised within a single _species_ (εîδος) ... are the real existences; but inasmuch as these individuals possess one common specific form, it will suffice to state the universal attributes of the species, that is, the attributes common to all its individuals, once and for all.’[48] This is surely not very far removed from the modern biological conception of a species. [48] _De partibus animalium_, i. 4; 644ᵅ 22. ‘But as regards the larger groups—such as birds—which comprehend many species, there may be a question. For on the one hand it may be urged that as the ultimate species represent the real existences, it will be well, if practicable, to examine these ultimate species separately, just as we examine the species Man separately; to examine, that is, not the whole class Birds collectively, but the Ostrich, the Crane, and the other indivisible groups or species belonging to the class.