એરિસ્ટોટલ's classification of living things contains some elements which still existed in the ઓગણીસમી સદી. What the modern zoologist would call vertebrates and invertebrates, એરિસ્ટોટલ called 'animals with blood' and 'animals without blood' (he was not to know that complex invertebrates do make use of haemoglobin, but of a different kind from vertebrates). Animals with blood were divided into live-bearing (humans and mammals), and egg-bearing (birds and fish). Invertebrates ('animals without blood') are insects, crustacea (divided into non-shelled – cephalopods – and shelled) and testacea (molluscs). In some respects, this incomplete classification is better than that of Linnaeus, who crowded the invertebrata together into two groups, Insecta and Vermes (worms).

For Charles Singer, "Nothing is more remarkable than [એરિસ્ટોટલ's] efforts to [exhibit] the relationships of living things as a "scala naturae"" એરિસ્ટોટલ's "History of Animals" classified organisms in relation to a hierarchical "Ladder of Life" ("scala naturae"), placing them according to complexity of structure and function so that higher organisms showed greater vitality and ability to move.

એરિસ્ટોટલ believed that intellectual purposes, i.e., formal causes, guided all natural processes. Such a teleological view gave એરિસ્ટોટલ cause to justify his observed data as an expression of formal design. Noting that "no animal has, at the same time, both tusks and horns," and "a single-hooved animal with two horns I have never seen," એરિસ્ટોટલ suggested that Nature, giving no animal both horns and tusks, was staving off vanity, and giving creatures faculties only to such a degree as they are necessary. Noting that ruminants had a multiple stomachs and weak teeth, he supposed the first was to compensate for the latter, with Nature trying to preserve a type of balance.

In a similar fashion, એરિસ્ટોટલ believed that creatures were arranged in a graded scale of perfection rising from plants on up to man, the "scala naturae" or Great Chain of Being. His system had eleven grades, arranged according "to the degree to which they are infected with potentiality", expressed in their form at birth. The highest animals laid warm and wet creatures alive, the lowest bore theirs cold, dry, and in thick eggs.

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