50 Bulletin American Museum of Natural History. [Vol. XXIII, 



PART II. CONCERNING THE POLYMORPHISM OF ANTS. 



There is a sense in which the term polymorphism is applicable to all 

 living organisms, since no two of these are ever exactly alike.* But when 

 employed in this sense, the term is merely a synonym of 'variation', which 

 is the more apt, since polymorphism has an essentially morphological tinge, 

 whereas variation embraces also the psychological, physiological, and etho- 

 logical differences between organisms. In zoology the term polymorphism 

 is progressively restricted, first, to cases in which individuals of the same 

 species may be recognized as constituting two or more groups, or castes, 

 each of which has its own definite characters or complexion. Second, the 

 term is applied only to animals in which these intraspecific groups coexist in 

 space and do not arise through metamorphosis or constitute successive 

 generations. Cases of the latter description are referred to 'alternation of 

 generations' and 'seasonal polymorphism.' And third, the intraspecific 

 groups which coincide with the two groups of reproductive individuals 

 existing in all gonochoristic, or separate-sexed Metazoa are placed in the 

 category of 'sex' or 'sexual dimorphism.' There remain therefore as 

 properly representing the phenomena of polymorphism only those animals 

 in which characteristic intraspecific and intrasexual groups of individuals 

 may be recognized, or, in simpler language, those species in which one or 

 both of the sexes appear under two or more distinct forms. 



As thus restricted polymorphism is of rare occurrence in the animal 

 kingdom and may be said to occur only in colonial or social species, where 

 its existence is commonly attributed to a physiological division of labor. It 

 attains to its clearest expression in the social insects, in some of which, like 

 the termites, we find both sexes equally polymorphic, while in the others 

 like the ants, social bees, and wasps, the female alone, with rare exceptions, 

 is differentiated into distinct castes. This restriction of polymorphism to 

 the female in the social Hymenoptera, with which we are here especially 

 concerned, is easily intelligible if it be traceable, as is usually supposed, to a 

 physiological division of labor, for the colonies of ants, bees, and wasps are 

 essentially more or less permanent families of females, the male representing 

 merely a fertilizing agency temporarily intruding itself on the activities of the 

 community at the moment it becomes necessary to start other colonies. We 

 may say, therefore, that polymorphism among social Hymenoptera is a phy- 

 sical expression of the high degree of social plasticity and efficiency of the 

 female sex among these insects. This is shown more specifically in two 



1 The term is used in tiiis sense bv Waxweiler, for example, in his admirable 'Esquisse d'une 

 Sociologie,' Brussels, 1906, pp. 141-143. 



