CREATION BY EVOLUTION
dry deserts on the one hand and in moist, tropical jungles and rain forests on the other. Many of the ants in these jungles and forests, owing to the seasonal drenching of the soil, build their nests in trees or inhabit the pith cavities of twigs and branches. We also observe that regions of the globe like Australia, which are inhabited by the most primitive mammals and birds (duck-bills, echidnas, marsupials, emus, etc.), are also inhabited by the most primitive ants (bull-dog ants of the genera Promyrmecia and Myrmecia), whereas countries like Europe and North America, which have highly specialized mammalian and bird faunas, are similarly inhabited by highly specialized and dominant ant faunas, with which, however, are intermingled a small number of primitive forms, which were once widely distributed but are now rare and are in process of extinction. Such a distribution can be explained only on the theory of evolution and is in complete agreement with all we know about the geological history and morphology of other organisms.
Conclusions from a comparative study of the habits of ants, or ant behavior, which is necessarily restricted to living forms, agree closely with the conclusions reached in the fields mentioned. Although all ants are social, they exhibit different degrees of social organization. This diversity is shown in different degrees of division of labor in the colonies as coordinated with their size and in differences shown by their component individuals. Thus among the most primitive ants many of the colonies are very small and the fertile females and workers are much alike in size and structure, but in the most highly socialized species (Dorylinae, Formicinae, and Myrmicinae) the colonies may be very large and the workers may be unlike the females and may even exhibit a differentiation of the worker into secondary castes, major and minor workers (Fig. 4), or soldiers and workers proper. Along
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