Sorting by what you can see
Around the 4th century BC Aristotle introduced the first scientific classification of living things. There are two main ways of classifying organisms — artificial and natural.
Artificial classification
External features and habitat and the like are taken into account, and the evolutionary relationships of the organisms are not represented by it.
Two examples
Grouped as ornamental plants, medicinal plants and poisonous plants.
Grouped as animals with wings and animals without wings.
Where the weakness shows
Under the criterion "animals with wings", both birds and insects are included. But birds and insects are two groups that evolution separated a very long way indeed.
A wing is not a wing
A bird's wing is made of bone and feathers; a butterfly's is a thin membrane. The two carry the same name because they do the same job, not because they are the same thing.
An artificial classification has many weaknesses. The next page has the way around them.