The human transport of animals across the Northern Atlantic 219 



new host, Solanum tuberosum, was enormously more abundant than the Solanum 

 rostratum of its original habitat in the Rocky Mountains.— Similarly, the European 

 vine (Vitis vinifera) was a more suitable host to the Phylloxera vitifoUae Fitch than 

 were the corresponding American species.— Species adapting themselves to in- 

 door conditions, especially in greenhouses, such as many Diplopods, Terrestrial 

 Isopods (Woodlice), &c., may gain thereby, not only an almost cosmopolitan 

 distribution, but also, due to an equal and favourable microclimate, a local abun- 

 dance widely surpassing that of their original outdoor habitat. 



However, these are exceptions. It seems reasonable to assume that normally 

 an animal or plant species is perfectly adapted to climatic and other conditions 

 prevailing in its natural habitat, that it finds its optimum where it developed (from 

 a phylogenetical point of view), usually in the central part of its distribution area, 

 and that a successful introduction by the aid of man into a foreign country means 

 that it by lucky chance entered a new area with approximately the same, and not 

 better, environmental conditions. Then this alone cannot be responsible for 

 subsequent super-abundance. 



The second explanation is based on the assumption that "competition" (excluding 

 the relations between predator or parasite and prey or host) is a major factor 

 determining the composition of fauna and flora of a given area, especially the assort- 

 ment of species within a micro-habitat. Without any doubt this is true as far as 

 sedentary organisms are concerned. Between species and individuals of higher, 

 autotrophic plants there is a continuous competition, not only for space, but also 

 for light and nutriment. It seems therefore defensible to explain the colonization 

 of waste ground in towns and ports of the Canadian East by an almost unmixed 

 European flora {vide above, p. 218) as at least partly due to the fact that practically 

 no corresponding ecological element of the indigenous flora was present when the 

 White Man arrived. 



In the second place, mammals and birds, with their higher-developed psychical 

 functions, especially their instinct for ownership of a wide area ("territory"; 

 Germ.: Revier) around their breeding places, certainly in their normal state live 

 under permanent (intra- as well as inter-specific) competition which is essentially 

 difi'erent from the blind struggle for life in most non-sedentary evertebrates. The 

 effect is that the population of for instance a certain bird species is many times 

 sparser than if it had been regulated by food supply alone. The rapid increase of 

 area and abundance of the English Sparrow (Passer domesticus L.) and the Starling 

 (Sturnus vulgaris L., fig. 4), both purposely introduced into North America, may 

 very well have been determined by the lack of indigenous competitors among birds 

 living in the immediate neighbourhood of man. 



Turning to the lower terrestrial fauna, the literature contains a good many 



