THE EVOLUTION OF ANTS
between similar species of wild organisms, for the process of racial development or evolution is so exceedingly slow that even some slight structural changes may have required millions of years, or at any rate periods far too long to fall under the observation of a being so ephemeral as man. The proofs of this very long historical evolutionary process are therefore indirect; they derive their value from the convergent and mutually corroborative inferences drawn from studies made in widely different fields of the great science of biology. At least five of these fields furnish significant historical inferences—the study of fossil animals and plants (palaeontology), the comparative study of the development and structure of existing forms (morphology, or anatomy and embryology), the study of the present geographical distribution of plants and animals (chorology), the study of the classication of plants and animals (taxonomy), and the comparative study of the behavior of animals (ethology). Obviously, the study of extinct or fossil species is of the greatest value, but the record of some species is deplorably fragmentary and most of the specimens found are imperfectly preserved. Although, therefore, all positive palaeontological data are precious, the fact that we have not yet found connecting or intermediate forms at particular geological horizons may be of slight significance. Comparative morphology and its shorthand expression, classification, are of enormous value in determining the possible genetic relationships between species, both living and fossil, and the distribution of living species as compared with that of their fossil allies is of great historical significance. Finally, the study of the behavior of existing animals and of the dependence of behavior on the structure and function of particular organs enables us to draw inferences in regard to the actual modes of life of their allied extinct species. After these very general
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