THE EVOLUTION OF ANTS
according to their structure we find that they fall into some seven subfamilies and that these may be most naturally regarded as seven large branches that arose from a single main trunk representing the most primitive and most wasp-like forms (Figs. 2 and 3). The existing species correspond to the green twigs and leaves at the tips of the branches of this "Stammbaum," or phylogenetic tree, and the new species that are discovered from time to time may be placed very naturally among their nearest allies according to this arborescent scheme. Now such an arrangement of the six thousand known Formicidae is the only one that will adequately represent the similarities or the relations of the forms, and the attempt to represent the morphological affinities of the species of any other group of organisms invariably produces the same kind of arrangement. This arrangement, moreover, would seem to admit only of a genetic or evolutionary interpretation.
It is, of course, impossible to give here any adequate account of the distribution of ants. With the exception of a few species that have been accidentally transported within recent times by man from one to another country, all ants are confined to rather narrow areas of the earth's surface, and their distribution agrees in general with that of other organisms, suggesting that the genera and species arose at different periods during geological time and then, with more or less modification, radiated to other regions, except as natural obstacles, such as large bodies of water or high mountain chains, may have prevented. The facts that most ants nest in the soil, that they avoid soil that is too constantly wet, that they are fond of warmth, and that they are abundant in certain arid regions suggest that they had their origin as a group on rather high continental areas during Mesozoic time. Many species, however, have since become adapted to life in
[ 217 ]
