PARASITE INTRODUCTIONS 393 



exemplified in the Hawaiian group. These conditions may be 

 summarised as follows, and they probably hold good for most 

 oceanic islands : 



(1) Warm and equable climate allowing parasites to multiply 

 unchecked by seasonal factors. 



(2) An indigenous fauna of a peculiar and restricted character, 

 evolved as the result of long isolation. Introduced parasites meet 

 with relatively little competition from indigenous forms, and the 

 parasitic element is but poorly developed. 



(3) The area to be covered by parasite colonisation is 

 circumscribed and, as there are only few main crops, adequate 

 organisation and centralisation of control experiments are greatly 

 facilitated. 



In most islands of continental origin parasite introduction has 

 to contend with a different type of environment. In Trinidad, 

 for example, conditions (1) and (3) hold good, but (2) is entirely 

 different. Owing to the geological formation of the island, and 

 its close proximity to the mainland of South America, the fauna 

 is essentially that of the neighbouring continent. Introduced 

 parasites consequently have to contend with a high degree of 

 environmental resistance of a biological character. Among a 

 fauna already rich in the parasite element new introductions, if 

 they are to be effective from the economic standpoint, require to 

 be species capable of forcing their way into and exerting influence 

 in already complexly adjusted biological associations. On the 

 other hand, in New Zealand, although (1) is far less ideal than in 

 Hawaii, the climatic conditions are more favourable than those 

 from which the introduced pests have come, and from where 

 parasite introductions have been made. Condition (2) has many 

 characteristics of an insular fauna, and although rich in many 

 dominant groups of insects, other groups are wanting or but poorly 

 represented in New Zealand, and the indigenous parasite and 

 hyperparasite element is not highly developed. 



In New Zealand there are, for example, according to Tillyard, no 

 indigenous Thysanoptera, only thirteen species of Odonata, and the 

 same number of Neuroptera ; Hemiptera number about 300 species 

 (Australia 1,970), and Hymenoptera include 316 species (Australia 



