xiv Introduction. 



drawn up excellent anal3^tical tables of species. In this 

 country Westwood, Halliday, Walker, and Fitch have 

 all done good work, and Mr. Peter Cameron \ in 

 his account of the Phytophagous Hymenoptera, just 

 completed for the Ray Society, has brought together 

 an immense amount of valuable material, new and 

 old. 



Dr. Adler began his observations on gall-flies in 1875, 

 and added certain new species described in these pages. 

 His important work, however, was the unfolding of 

 their life-history ; proving that while many species were 

 linked together in alternate agamous and sexual genera- 

 tions, others remained wholly agamous. 



Among biologists theories of dimorphism and meta- 

 genesis had been discussed in connexion with the 

 Cynipidae and were current as early as 1865 - ; in 1873 

 Bassett announced his theory of seasonal dimorphism ^ ; 

 and in the same year Riley established the existence 

 of alternation of generations between Cyntps operator 

 and Cynips operatola^. The numerous publications of 

 Thomas and Frank have cleared up man}^ special 

 points, and Beyerinck ^ in an admirably illustrated 

 monograph, full of original work and careful reasoning, 

 has added much both from a botanical and zoological 

 point of view. 



The existence of alternating generations in living 



^ Peter Cameron, A Monograph of the British Phytophagous 

 Hymenoptera, 4 vols. 1882-1893. Ray Society. 



^ Reinhard, ' Die Hypothesen iiber die Fortpflanzungsweise bie den 

 eingeschlechtigen Gallwespen,' 5^r/m. £'«^. Zeitschr. vol. ix. 1865. 



■• Bassett, Canadian Entornol. vol. v, pp. 91-94, May, 1873. 



* Riley, American Naturalist, vol. vii. p. 519, 1873. 



* Beyerinck, Dr. M. W., ' Beobachtungen iiber die ersten Entwick- 

 lungsphasen einiger Cynipidengallen,' Natnurk. Verh. der KoninkL 

 Akademie, Deel xxii. 



