Increase in Number of Species 169 



only one at a time in any one locality, they would mate with a local 

 member of the island species rather than have an opportunity 

 to mate with each other. Even if an odd family of vagrants became 

 established, their progeny would be rendered ineffective by cross- 

 mating with members of the island species. At this stage the parent 

 species would become re-established as a second invasion on the 

 island only if a small band reached it and if the band kept together 

 as a breeding unit. 



( 5 ) When the divergence between the island and parent species 

 became sufficiently great that a state of sexual isolation prevailed 

 between them, the situation would revert to stage 1, when a pair 

 of the parent species could start a continuing second colonization 

 of the island. 



There would be a period embracing all of stages 1 through 4 

 when sporadic vagrants could have reached the island without 

 leaving any tangible trace. If a "double invasion" were ultimately 

 accomplished, the record would give evidence only of the first 

 and last arrivals. 



The series of steps listed above are patterned after the require- 

 ments of animals such as birds or termites in which both the male 

 and female must be present during the breeding season. The situa- 

 tion would diflfer in certain respects in the case of animals such as 

 other insects, snakes, or mammals in which a vagrant gravid female 

 could establish a family. With these the more important factor 

 would be population size. A large island population would tend to 

 swamp the relatively minute proportion of vagrant progeny either 

 by amalgamation into the population or, if hybrid progeny were 

 sterile, by exterminating them because of chance cross-mating. 



Theoretically this series of colonizing events could be repeated 

 again and again, giving rise ultimately to many colonial species. 

 E. O. Wilson (1959) postulated that such multiple invasions have 

 been an important factor in the evolution of the ant fauna of 

 Melanesia. 



Continental reinvasion. It is tacitly assumed by Mayr (1942) and 

 others in the cited examples of the double invasion of islands, that 

 the species common to both continent and island was the parent 

 and that the species on the island was the offspring. This view is 

 emphasized even more strongly by those who believe that such 

 insular species become "less well adapted" than their continental 

 parents (Brown, 1957) and are therefore unable to invade the 

 continent successfully. In all cited examples of double invasion, 

 there is no evidence to contradict the difl^erent interpretation that 



