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 ade- 

 quately represent the similarities or the relations of the 

 forms, and the attempt to represent the morphological affini- 

 ties 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 evolu- 

 tionary 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 moun- 

 tain 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 



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