The human transport of animals across the Northern Atlantic 199 



pure chance too, provided that the species in question regularly occurs on places 

 where ships took ballast. This is not so. From the account above it is evident that 

 certain groups of animals, for instance Carabid beetles and "Iso-Myriapods", 

 have crossed the Atlantic, or at least made successful settlements on the other 

 side, more often than plant bugs {Hemiptera Heteroptera), Spiders [Araneae) and 

 others. This, of course, is not due to their taxonomic position but to one or more 

 characteristic features of their natural history. 



By grouping the animals found on the English ballast-places according to such 

 principles, instead of taxonomically, it would be possible to fix the foremost 

 qualifications favouring transport with ballast. The necessary analysis of the ecolog- 

 ical requirements and of certain other biological properties of the species was 

 carried out for all Coleopterous insects observed on the ballast-places (table 6). 



The biological and ecological properties recorded in the 7 columns of table 6 

 are valid for the fullgrown insect (imago), the larva in several species having a 

 diflFerent biology. The method used is defensible from the point of view that the 

 imago (especially a fertilized female) is far more important for a successful coloni- 

 zation of an oversea area than is the larva. 



Let us first consider the columns of table 6 separatedly. 



1st column. The planticolous species (p) constitute 28 per cent (48 species) of 

 the 174 Coleoptera from the ballast-places not introduced into North America, 

 but only 19 per cent (13 of 68 species) of the "emigrants". Terricolous species are 

 favoured, as has already been pointed out by Brown (1940, 1950). Similarly, 

 Molluscs and Iso-Myriapods are strongly represented among the introduced 

 forms, whereas very few of the many planticolous Hemiptera and Araneae (true 

 spiders) were carried over. Planticolous forms were less likely to be taken on board 

 with the material generally used- as ballast (above, p. 164) and, if this happened, 

 they ran the risk of being blown overboard due to their habit of climbing. 



2nd column. The hygrophilous species (h) constitute 25 per cent (44 species) 

 of the "non-emigrants", but only 6 per cent (4 species) of the "emigrants". 

 Xero- and mesophilous species are favoured, partly because dry ballast was more usual, 

 partly because there was considerable risk of desiccation during the long voyage 

 or, if the ballast became wet, the cause was usually salt water, which is injurious 

 to normal hygrophilous insects. 



3rd column. The "emigrants" and the remaining species were distributed on 

 the three investigated kinds of soil as follows: 



waste ground pure sand saline localities 



"Emigrants" 61 species = 90 % 6 = 9% 1=1% 



Others 1 30 ,, - 75 % 14 = 8 % 30 = 17 % 



