In the fine optimistic years of early evolutionary paleontology, it became fashionable to draw up genealogical trees for various groups of animals, and especially of mammals, in which the supposed interrelationships of all these creatures were shown by representing the living animals as the ends of twigs and their ancestors through preceding ages as lower and connected parts of the stems, branches, and trunk of the tree of life. It was generally recognized that certain details remained to be filled in, but on the whole the concept was simple and satisfying, and much of the work of that period gives the impression that the missing details were not considered important and that their discovery was considered imminent.

The use of such graphic means of indicating descent is still common, and there are few paleontologists who have not constructed some sort of phyletic tree, either in words or in . . .

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