THE ORIGIN OF SPECIES 



He attributes the absence of hybrids among the former to the effect ot 

 differences in behavior pattern which may break the "engagement" of 

 pairs which are not of the same species. 



Spieth has made a detailed study of the sexual behavior of the six spe- 

 cies of Drosophila. He found that courtship and mating can be divided 

 into six phases, at any one of which incompatible behavior of the potential 

 mates may break off the courtship. Although several hundred attempts 

 were made to obtain interspecific crosses, only once did actual copulation 

 result. In the majority of cases, courtship was stopped during the first 

 stage. Yet the observable differences in the courtship patterns of the 

 several species are rather minor, being concerned with tapping of the 

 female by the male, postural reflexes, and similar movements. In other 

 species, however, differences in courtship patterns may be more pro- 

 nounced. In crabs of the genus Uco, species can be recognized at a dis- 

 tance by their motions during courtship. Birds which show only minor 

 morphological differences may be easily recognized by their songs. The 

 mating "dances" of salamanders and turtles may be very striking. 



Scents, songs, and recognition marks perhaps all belong here. It is well- 

 established that scent plays a major role in mating reactions of the Lepi- 

 doptera. If a female of a rare species is placed in a screen cage, many 

 males will gather around it soon after the cage is placed outdoors. But 

 if the female is instead exposed in a glass container, the males do not 

 assemble. That these scents are highly specific can be shown by the 

 selective response of males when two closely related species of female are 

 put out in the same area. "Wrong" associations rarely occur. Petersen 

 studied a case in which no less than 37 species of a single genus of moths 

 live in a single valley without interbreeding. Visible differences between 

 these species are minor, and Petersen believes that conspecific matings 

 are guaranteed by the scents of the moths. 



Songs of birds are well known for their role in mating, and many in- 

 stances are known in which the sounds produced by insects also play the 

 same role. In addition to the recognition of conspecific mates, these may 

 serve to stimulate sexual activity. As pointed out above, the wing move- 

 ments of the male of Drosophila serve to hasten the receptiveness of the 

 female, but once excited, she will accept a ivingless male. Mayr believes 

 that the many phenomena formerly described as sexual selection may be 

 properly understood simply as sex-stimulating mechanisms. This is very 

 plausible, and if correct would immediately bring these phenomena back 

 into accord with the general theory of natural selection. 



Mechanical factors were once regarded as important isolating mecha- 

 nisms, but it now appears that their importance was overrated, if not 

 completely in error. Among insects, the morphology of the genitalia may 

 be very complicated, and it frequently presents the best available taxo- 

 nomic characters. On tliis basis, Dufour long ago proposed the "lock and 

 key" theory, that there must be a very exact correspondence between the 

 morphology of male and female parts to permit copulation. The female 

 genitalia are thus compared to a lock which can be opened only by one 

 key, namely the male genitalia of the same species. This theory is very 



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