74 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, I46 



occurs, and loses this advantage when it becomes more widely 

 prevalent, it is said to be a balanced polymorphism. However, when 

 such a form appears to be without obvious selective value or adaptive 

 advantage, it is termed a neutral polymorphism. It is not clear that 

 the distinction between neutral and balanced polymorphism is bio- 

 logically factual ; it might be better to say that the neutrality of 

 certain balanced polymorphisms is merely an observational inference 

 rather than an established genetic fact. It is in this restricted sense 

 that the situation in Clamator may be described as neutral. In the 

 light of present evidence, at least as far as Clamator jacobinus is 

 concerned, the "normal" morph and the black phase seem equally 

 well adapted to their common environment. The birds interbreed 

 freely and act as though they recognize no differences between them, 

 and furthermore the black form is not noticeably spreading geo- 

 graphically or becoming increasingly numerous where it occurs to- 

 gether with the white-breasted form (and it is not known to occur 

 anywhere as the sole morph). In some localities it is apparently as 

 common as, and in a few spots even more numerous than, the pale 

 morph, but there is no evidence to suggest that the ratio has changed 

 appreciably in the past half century or more of observations. It must 

 be admitted that this absence of evidence is not nearly as good a 

 support as would presence of negative evidence have been. The 

 "evidence," if it may so be termed, is chiefly the memory and rec- 

 ollections of observers of long residence, unsupported by critical 

 records and notes. 



In the case of C. levaillantii we can only assume that the situation 

 is also one of neutral polymorphism, as we do not have the direct 

 evidence available in C. jacobinus. It seems, however, a safe assump- 

 tion. 



Polymorphism is an expression of gene frequency, and neutral 

 polymorphism implies a fairly stable frequency picture. Inasmuch 

 as the occurrence of melanistic morphs of both levaillantii and of 

 jacobinus away from their geographically restricted areas of de- 

 veloped neutral polymorphism is so sporadic and infrequent, it follows 

 that these two wide-ranging species, each with geographically con- 

 tinuous, uninterrupted, nonfragmented distribution patterns, have 

 local populations whose gene pools seem to be fixed and seem to be 

 kept unavailable to adjacent populations of their own kind.- There 



2 In the case of C. jacobinus this statement is intended to cover only the 

 African part of its range ; its extensive Asian population is of course effectively 

 cut off from the larger African one. No black-phase birds of this cuckoo have 

 ever been noted in Asia (from which area I have examined at least 200 examples, 

 as well as read and checked the many observations and records in the literature). 



