186 Increase in Number of Species 



exactly the same situation as N. maurus (Ghent and Wallace, 

 1958). 



It is possible that both N. maurus and IV. hetricki evolved through 

 geographic isolation and that their changed seasonal adjustments 

 occurred during that evolution. The circumstances of both cases, 

 however, suggest the greater probability of evolution through 

 sympatric temporal isolation. 



Bigelow (1958) based his similar concept of temporal isolation 

 on two closely related species of field crickets of the genus Acheta. 

 In A. pennsijlvanicus winter is passed in the egg stage, and in A. 

 veletis it is passed in the late nymphal stage. As a result there is 

 extremely little overlapping in the period of adult emergence of the 

 two species. On the basis of laboratory rearings the two species 

 appear to be completely isolated sexually. Bigelow ( 1958 ) and 

 Alexander and Bigelow (1960) explained the evolution of these 

 two cricket species in the same manner as that employed to explain 

 the sawfly examples in Neodiprion. They give the name allochronic 

 speciation to the process. 



HOST ISOLATION 



In practically all groups of host-specific parasites or predators, a 

 phylogenetic line established on one host has either changed to a 

 new host or given rise to a daughter line on another host. Well- 

 known examples abound in the insects. A rodent-infesting group 

 of fleas has become established on burrowing owls (Rothschild 

 and Clay, 1952), the moth Platysamia coliunbia evolved on larch 

 from a parent line feeding only on angiosperm hosts (Sweadner, 

 1937), and two species of the sawfly genus 'Nematus transferred 

 independently to the legume host Robinia from an ancestor utilizing 

 a willow host. Dethier (1954) gives other examples, and Mayr 

 ( 1947 ) and Emerson ( 1949 ) cite examples from other groups of 

 animals and from parasitic plants. The physiological and genetic 

 bases of this host transfer are poorly understood (Dethier, 1954), 

 and the genetic situation is especially obscured because the best 

 explanation for the meagre observed data is some type of pheno- 

 typic habituation. 



In certain host transfers of this type, the possil3ility is very real 

 that one species gives rise to a daughter species on a new host 

 within the geographic and general ecological range of the parent 

 species. As Mayr (1942, 1947) pointed out in opinions opposed to 

 giving credence to this type of sympatric species fission, the host 

 transfer would have to associate interbreeding males and females 



