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Psyche 
[March 
precocious decline of juvenile hormone titer in the maturing larva, or 
if on the other hand as Brian (1959, 1961) proposes, to a sharp rise 
in the concentration of ecdyson near the critical period or periods of 
caste determination in larval ontogeny, it seems conceivable that 
mutations have accumulated in evolution affecting neurosecretory 
products or processes or timing, such that the threshold for worker- 
queen determination is passed only rarely in species of the R. metallic a 
complex, and never in many of the larger species. Alternatively, it is 
conceivable that dimorphic female reproductive forms originally 
existed, as they presently do, for example, in species of N eophyrcaces, 
one form being ergatoid and the other unmodified, and that further 
evolution resulted in the loss of the latter and so close an approxi- 
mation of the worker form, by the former that it is no longer dis- 
tinguishable except through its reproductive capacity. If the latter 
course has been the actual one, however, we should perhaps suspect 
that the fertile “workers” would be comparatively rare in the Rhyti- 
doponera colony, at least as rare as are the laying true queens in 
colonies of relatively primitive pleometrotic species. It appears, as 
will later be indicated, that they are in fact much more abundant. 
Whatever the channel of evolution at the level of individual 
morphology, its end result poses some intriguing questions about the 
direction of evolution in Rhytidoponera at the level of the society. 
Some years ago Sturtevant (1938), and later Williams and Williams 
( 1 95 7 ) > emphasized the evolutionary significance of the close family 
relationships which typically obtain among the members of the 
matrifilial colony so characteristic of the higher social Hymenoptera. 
Very recently W. D. Hamilton (1964), in two important papers, 
beginning with Haldane’s (1955) concept of the evolutionary sig- 
nificance of genetically based altruistic behavior, has derived a quantity 
in social evolution that he defines as “inclusive fitness.” It may be 
regarded in certain respects as an analogue at the social level of the 
concept of Darwinian reproductive fitness at the level of the individu- 
al. Like the latter it should be found to maximize in evolution. This 
maximization, in the social insects, has obviously involved an extra- 
ordinary evolution of worker altruism, at both structural and 
behavioral levels. Now as Hamilton demonstrates, it can be expected 
that such an evolution will be positively correlated with the 
maintenance of close genetic relatedness among the members of a 
colony. In the absence of parthenogenesis, the closest genetic relation- 
ship between queen and worker possible in a colony is that of mother 
to daughter, and among workers that of siblings. The pronounced 
evolutionary trends toward the inclusion of but two generations in the 
