Again, independently of nomenclature, “while Aristotle knew a fairly large number of animals, the notion of grouping them in definite order, which should express their greater or less degree of similarity, does not appear to have presented itself to his mind. Hence he did not attempt what we call classification. He compares different animals together, by every possible means, and endeavors to reduce the result of his comparisons to general propositions.” In this way he arrives at relations which are sometimes important, sometimes without importance. For example: among animals, some have blood, some lymph, which takes its place: this division, notwithstanding the error on which it is based, corresponds broadly speaking with the distinction between vertebrates and invertebrates. Animals “which have blood” are subdivided into viviparous and oviparous. Further, animals that fly are ranged in three categories, according as their wings are feathered (birds), or formed by a fold of skin (bats), or dry, thin, and membranous (insects). Then there is a division of animals into aquatic and terrestrial, social and solitary, migratory and sedentary, diurnal and nocturnal, domestic and wild, etc. In sum, there is co-existence of two processes: one scientific, implying a preliminary analysis; the other of common observation, which does not sensibly differ from concrete-abstract classifications; and the idea of a hierarchy formed by abstraction of abstracts, by a systematic arrangement of the animal kingdom, has not yet made its appearance. Yet Aristotle’s work, just by reason of its composite nature, is interesting to the psychologist who studies the evolution of the faculty of abstracting and generalising. We may pass over two thousand years, during which no progress was made, till we come to Linnæus. “He was the first man who distinctly conceived the idea of expressing, under a definite formula, what he believed to be the system of nature.” His nomenclature is fixed. Under the names of classes (_genus summum_), orders (_genus intermedium_), genera (_genus proximum_), species, varieties, he proposes subdivisions of decreasing value, embracing a greater or less number of animals which all present in common more or less general attributes. He pursues the research after fundamental characteristics, and _essential_ similarities, incessantly correcting his first results. Thus it is only at the eleventh edition of his _Systema naturæ_ that the class of “Quadrupeds” is converted into Mammals: the Cetacea are included in this class, and no longer placed among the fish, as also bats, which were formerly classified with birds, etc.[79] Whatever their objective value, we have here a true system of rational concepts.