CoachTo sort a collection is to put together the things that go together. A criterion is the rule used for the sorting — “has six legs”, “has feathers”. A good criterion for living things is a feature of the body you can observe: something the animal or plant has, not the place it lives or what it happens to be doing.
Examples
Example 14.2 (Two sortings of the same animals)
Take the cat, the duck, the trout and the ladybug. Sorted by “lives near water”: duck and trout together. Sorted by “has feathers”: the duck stands alone. Both are sortings — but only the second uses the animal’s own body, and that is the kind biologists trust: an animal can move house, but it cannot change its feathers for scales.
Example 14.5 (Surprises done right)
Criteria save us from lazy guesses. The bat flies like a bird — but it has fur, no feathers, and feeds its young milk: a mammal. The whale swims like a fish — fur reduced to almost nothing, but milk for its calf and no scales: a mammal too. The ostrich cannot fly at all, yet feathers and beak say: bird. The criterion, not the impression, decides.