Introduction

Look around at the animals you come across in everyday life — a stray dog, a sparrow, an earthworm turned up by a spade, a butterfly in the garden — and it is obvious how different they are from one another in body structure and outward form. This diversity is not a handful of exceptions; biologists have already described well over a million distinct animal species, and the true number is almost certainly larger still. Faced with variety on this scale, some systematic way of organising and grouping animals stops being a convenience and becomes a genuine necessity — it is what makes it possible to study animal life at all without having to consider every species separately.

Classification also serves a second, ongoing purpose: whenever a new species is discovered and described, classification is what allows it to be assigned a definite, systematic position among the animals already known, rather than being left as an isolated curiosity.