The human transport of antmals across the Northern Atlantic 221 



in Europe, and this in spite of the fact that, in abundant species, intra-spGcific 

 competition (between individuals of the same species) must be considered stronger 

 than interspecific competition (between the individuals of different though pos- 

 sibly near related species). In my opinion, terrestrial habitats very seldom and 

 then for only short periods possess a "saturated" fauna and therefore competition, 

 in the limited sense here used, hardly constitutes a governing factor for lower 

 animals without instincts of "ownership", except in cases where they are highly 

 specialized, for instance in choice of food. 



The third explanation (above) contains the most obvious facts contradicting 

 the opinion that as a rule reduced competition is responsible for the often extra- 

 ordinarily rapid dispersal of an introduced animal species, above all embodied in 

 the method of ''biological control'' against this special form of insect pests. This is 

 based on the assumption that the abundance of a species in its native country is 

 balanced by natural enemies (predators, parasites and diseases) from which it partly 

 or entirely escapes in a new area. By careful observations in the original patria, 

 the most important of these natural enemies may be discovered, artificially repro- 

 duced and finally released in the infested country. 



The best examples of success in biological control of this kind emanate from 

 the United States. In that country, due to the extraordinarily high number of 

 introduced culture-plants, the demands for proceedings against imported pests 

 have been greater than anywhere else, and likewise the economical powers to 

 carry them out. 



The following examples refer to insects unintentionally or purposely exchanged 

 between Europe and North America and are selected from Howard (1930), Essig 

 (1931), "Handbuch der Pflanzenkrankheiten" (1949-54), and other current eco- 

 nomic-entomological literature. 



A. Species of European origin 



Galerucella luteola O. F. Miill. (xanthomelaena Schrk.) (Elm Leaf Beetle). Intro- 

 duced as early as the i83o's, now over the main part of U.S.A. and very noxious. 

 A species of Tetrastichus (fam. Chalcididae), parasiting the eggs in Europe, and a 

 few Tachinid flies, have been tentatively introduced. 



Phytonomus punctatus F. (Clover Leaf Weevil) and Ph. posticus Gyll. {variabilis 

 Hbst.) (Alfalfa Weevil) {vide map, fig. 6).^ The latter has been successfully con- 



^ Flight of Phytonomus posticus has been frequently observed in U.S.A. It is therefore 

 possible that macropterous homozygotes have been introduced by which the dispersal 

 would be considerably favoured. In Europe, the species is dimorphic with respect to the 

 hind-wings. 



