While this sounds simple, in actual practice it may be quite difficult. First of all, we have to decide what kind of similarities are the most important for our purpose. One of the earliest classification schemes placed in one category all those organisms which lived in the same habitat. Thus fish, whales, and penguins were classified as swimming creatures. This type of classification was often based on the principle that creatures possessing analogous organs should be classified together.
Analogous organs are organs that have the same function.
The fins of fishes and the flippers of whales and penguins are analogous organs because they are all used for swimming. The wings of birds, bats, and insects are analogous organs that make flying possible. As more knowledge was gained about the anatomy of living things, it became apparent that similarities of habitat and of analogous organs were often rather superficial. The fact that bats have fur and nurse their young, birds have feathers and lay eggs, while insects are cold-blooded and have no internal skeleton suggested that these organisms differ from one another in more important ways than they resemble one another. An appreciation of the truly significant ways in which organisms resemble or differ from one another enabled the Swedish naturalist Carolus Linnaeus to found the modern system of classification.