Wasps, Bees, and Ants 559 



It is evident, therefore, says Wheeler, that the ants of this genus 

 have originally possessed certain traits which made it specially easy for 

 them to enter into symbiotic relations with other species of ants. Some 

 of these fundamental or original traits may still be recognized in the genus, 

 to wit: 



"i. The genus has a very wide geographical distribution, a prerequisite 

 to the establishment of such numerous and varied relations with other ants. 



"2. The species are all of small size. This must undoubtedly facilitate 

 their association with other ants. 



"3. The colonies consist of a relatively small number of individuals. 

 This, too, must greatly facilitate life as guests or parasites in the nests of 

 other ants. 



"4. Most of the species are rather timid, or at any rate not belligerent. 

 They are, therefore, of a more adaptable temperament than many other 

 ants even of the same size (e.g., Tetramorium cosspitum). Forel has 

 shown that L. iubero-affinis will rear pupae of L. mylanderi and even of 

 Tetramorium ccespitum and live on good terms with the imagines when they 

 hatch. 



"5. There, is no very sharp differentiation in habits between the queens 

 and workers of Leptothorax. This, too, should facilitate symbiosis. The 

 queens, as I have shown in the case of L. emersoni, may retain the excavating 

 instinct and the instincts which relate to the care of the larvae. 



"6. The similarity in instinct between the queens and workers of Lepto- 

 thorax finds its physical expression in. the frequent occurrence of interme- 

 diate or ergatogynous forms. So-called microgynic individuals, or winged 

 queens no larger than the workers, have been frequently observed by Forel 

 and Wasmann in L. acervorum. Those observed by the latter author also 

 showed color transitions between the normal queens and workers." 



Finally, Wheeler points out that this heterogeneity of habit and these 

 existing gradatory steps between strictly non-working fertile queen and 

 strictly non-fertile working-worker, are evidence for the selection theory as 

 explaining the division of labor and differentiation of structure in the special- 

 ized ant communities. "Viewed as a whole, these different symbiotic rela- 

 tions cannot be said to bear the ear-marks of internal developmental causes 

 operating in a perfectly determinate manner. Indeed, appearances are 

 quite otherwise and seem rather to point to indeterminate variations which 

 have been and are still in process of being seized on a fixed by natural selec- 

 tion. It must also be admitted that the same appearance is presented by 

 the whole complex of conditions in compound and mixed nests, but the 

 demonstration is more cogent when it can be shown that we have relations 

 as different as those of dominant species (L. emersoni) and slaves (L. acer- 



