246 I The Process of Evolution 



ornithological investigations, such differentiation may be present 

 but undetected. There also seems to be no mechanism that could 

 prevent interchange of genetic information in places where several 

 gentes occur together, especially in view of the rather large ter- 

 ritories occupied by the males. 



The resemblance between the cuckoo egg mimics and the series 

 of mimetic polymorphs found in females of Papilio dardanus is 

 largely superficial. The major difference lies in the necessity for the 

 female cuckoos to lay their eggs in the nest of the proper host spe- 

 cies. (The choosing of the proper nests by the female cuckoos may 

 be explained by the phenomenon of imprinting. ) It is not sufficient 

 for a population to be genetically structured so that a multiplicity 

 of distinct forms is produced; the structure must be such that the 

 proper egg type is laid by a bird which chooses a particular foster 

 parent. Thus if the female cuckoo raised in the nest of a reed warbler 

 were to be inseminated by a male of the gens parasitizing the white 

 wagtail, her eggs might be better mimics of white wagtail eggs than 

 of reed warbler eggs. She would continue to lay her eggs in the 

 "wrong" nest, and in all probability her young would have a rela- 

 tively low chance of survival. Unfortunately, nothing is known of the 

 genetics of cuckoo egg color, and so the importance of the male con- 

 tribution is conjectural. 



It seems likely that, in both cuckoo gentes and Papilio dardanus 

 mimetic polymorphs, differentiation may have taken place in isola- 

 tion. When two differentiated isolates of the butterflies came into 

 contact in areas where both models were present, disruptive selec- 

 tion established a polymorphic system (as described in Chap. 7). 

 However, when two cuckoo gentes, each parasitizing a different host, 

 came into contact in an area occupied by both hosts, another factor 

 was probably added. In addition to disruptive selection (lower re- 

 productive success of genotypes producing eggs intermediate be- 

 tween the hosts), there was probably also selection favoring be- 

 havioral patterns that encouraged positive assortative mating (pair- 

 ing of individuals raised by the same foster parents ) . This seems to 

 have been accomplished most successfully where the host forms 

 have somewhat different ecological requirements, the male cuckoos 

 remaining in the familiar habitat where they were reared and con- 

 sorting with females of like background. 



In places where the habitats of the host forms tend to overlap, 

 intermediate-type eggs are often laid and the mimicry breaks down. 

 The gentes of cuckoos seem to be somewhat intermediate between 

 polymorphic forms and what might be called host races. The degree 

 of perfection of the mimicry apparently is dependent on the degree 



