Simple-minded people, who do not feel the need of precision in matters of natural history, have in all ages divided animals into four-footed beasts which walk on the earth, birds which fly, fishes which swim, and perhaps reptiles which creep. This is the classification found in the Babylonian and Hebrew narratives of the great flood. Plants they naturally divide into trees and herbs. It was not very long, however, before close observers became discontented with so simple a grouping. They discovered that the bat is no bird, though it flies; that the whale is no fish, though it swims; that the snake comes nearer in all essentials to the four-footed lizard, and even to the beast of the field, than to the creeping earthworm. At a much later time they discovered that pod-bearing or rose-like herbs may resemble pod-bearing or rose-like trees more closely than all trees resemble each other. Moreover, a multitude of animals became known which cannot be classed as either beasts, birds, fishes, or reptiles, and a multitude of plants which cannot be classed as either trees or herbs. [Illustration: BIRD'S SKELETON. For comparison with human skeleton (opposite), lettered to show the answerable bones. From Belon's _Book of Birds_, 1555.] [Illustration: HUMAN SKELETON. For comparison with bird's skeleton (opposite), lettered to show the answerable bones. From Belon's _Book of Birds_, 1555.] Aristotle found himself obliged to rectify the traditional classification of animals in order to remove gross anomalies. When learning decayed the traditional classification came back. Thus the _Ortus Sanitatis_ (first published in 1475, and often reprinted) adopts the division into (1) animals and things which creep on the earth; (2) birds and things which fly; (3) fishes and things which swim. No consistent primary division of plants was proposed by Greek or Roman, nor by anyone else until the seventeenth century A.D. This conflict of systems should have raised questions concerning the nature of classification and the relative value of characters. Some of the most striking resemblances found among animals and plants are only superficial; others, though far less obvious, are fundamental. Whence this difference? Why should scientific zoology make so little of the place of abode and the mode of locomotion; so much of the mode of reproduction and the nature of the skeleton? The answers were vague, and even the questions were rare and indistinct. But a metaphorical term came into use which was henceforth more and more definitely associated with fundamental, as distinguished from adaptive, likeness. Such likeness was called _affinity_,[1] though no attempt was made to explain in what sense the term was to be understood. As late as the year 1835 one of the first botanists in Europe (Elias Fries) could say no more about affinity between species than that it was _quoddam supernaturale_, a supernatural property.