CREATION BY EVOLUTION 



very slowly and imperceptibly. Probably the same was true 

 of the ants of periods antedating the Tertiary, though there 

 may have been in those periods occasional spells of accelera- 

 tion and efflorescence of new forms. 



When we carefully study the anatomy and development of 

 the various species of ants we find that they are essentially 

 wasps, and that they are closely allied to species of certain 

 existing families of wasps, the Tiphiidae, Mutillidae, and 

 Thynnidae. We must, indeed, suppose that the ancestors of 

 these families produced also the ants, the Formicidae. But 

 the members of these families, like most other wasps, are 

 solitary, and, like most animals, possess only a single type of 

 female; whereas among the ants each species presents two 

 female phases, or castes, one of which, the "queen," is fertile 

 and nearly always winged, and the other, the "worker," is 

 always wingless and nearly always sterile. In only a few 

 species of ants, and those highly parasitic species, do we find 

 no worker caste. There is every reason to assume that in 

 these species the worker has been lost or suppressed within 

 comparatively recent time. We must therefore conclude 

 that sexual trimorphism — that is, the presence in each species 

 of three castes, male, fertile female, and sterile female, or 

 worker, which were perfectly developed also in the known 

 fossil ants of Tertiary time — ^was first established among the 

 Mesozoic ancestors of the family Formicidae. A similar tri- 

 morphism has arisen independently among the social bees 

 and social wasps, but it has evidently been of much more 

 recent development, for among these insects the worker is 

 much more like the fertile female and always has wings. 

 Then, too, the differentiation of fertile and sterile females 

 among certain tropical wasps is so feeble that the evolution 

 of the two castes may be said to be still uncompleted. 



When we arrange all the species of living and fossil ants 



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