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ZOOLOGY: W. M. WHEELER 
primitive wasps belonging to the families Scoliidae, Mutillidae and 
Thynnidae, but authorities differ as to which of these families should 
be selected as the most probable ancestors. Emery^ believes that the 
ants arose from the Mutillidae, Forel from the Thynnidae and Hand- 
lirsch^ from the Scoliidae. But as all three families are so closely re- 
lated to one another that authorities fail to assign them the same limits, 
the differences of opinion are after all not very pronounced. 
HandKrsch^ advances the opinion that the ants first made their ap- 
pearance during the Cretaceous, but I am incHned to seek their origin 
in an earHer period, during the Jurassic or possibly even as far back as 
the Triassic. According to Schuchert,^ these were periods of maxi- 
mum continental emergence and aridity and would therefore present 
what I conceive to be the optimum environmental conditions for the 
development of the family Formicidae. The insects most closely re- 
lated to the ants (Thynnids, Scoliids, MutilHds) are very decidedly 
xerothermic and hence confined to deserts or to hot sandy and gravelly 
situations, and the ants present a number of peculiarities which seem 
to indicate more or less clearly that they originally Hved and developed 
in the same kind of a habitat. They are at the present time extra- 
ordinarily abundant in species and individuals in the desert regions 
of the globe (Australia, North Africa, Sonoran Regions of North America) 
and as a group seem to show in their inherited small average stature 
the stunting effects of an arid environment. The great majority of 
species have retained the primitively terrestrial and fossorial habit, 
which is an obvious adaptation to avoiding intense heat, insolation and 
evaporation during the summer months and the cold of nights and of 
the winter. The aptery of the workers and dealation in the females 
are closely connected with such habits. Most of the species, moreover, 
are decidedly petrophilous and many are exquisitely hypogaeic. The 
marriage flight, to which the males and females of most of the species 
so rigidly adhere, would seem to be a habit that had originated in open, 
unobstructed country. The adaptations, though numerous and intri- 
cate, to living in mesophytic and hygrophytic forests (Amazonian and 
East Indian rain-forests) are clearly secondary and of much more re- 
cent origin. 
That the workers of ants originally possessed wings like the females 
is shown by the presence of minute vestiges of these organs in the larval 
and pupal stages® and by the occasional, pathological development of 
very small wings in the adult. This condition occurs in the 'pterergates' 
found by Donisthorpe^ and myself^ in certain species of Myrmica, Cryp- 
tocerus and Lasius. The development of aptery, with the concomitant 
