How were animals grouped together in Aristotle's system of classification?
Aristotle divided animals into "those without blood" and "those with blood," a dichotomy that roughly corresponds to the modern classifications of vertebrates and invertebrates. He divided animals with blood into categories based on how they gave birth, i.e. to live young or by laying eggs. Animals without blood included mollusks, insects, and crustaceans. Within these broad categories, he recognized different species, which he claimed were evidence of a "chain of being" with human beings at its pinnac;le. The important thing is that Aristotle classified creatures based on their physical characteristics and behaviors, an approach that has been profoundly influential on more modern taxonomic systems.