2i8 CHAPTER II 



on the Atlantic as well as on the Pacific side, partly because the Indian agriculture 

 was very primitive there, partly because there was no direct connection with the 

 central" prairie enabling steppe plants to invade the artificial clearings. Conse- 

 quently, when the White Man first arrived and permanent settlement founded upon 

 agriculture started, there were few indigenous plants present able to intrude as 

 weeds. This gave free entrance to the corresponding floral element from Europe 

 which, almost free from competition, soon became dominating on cultivated and 

 waste ground, above all within the northern half of the Atlantic coast. Around the 

 ports of Newfoundland and Nova Scotia it is still easy to find acres of land in- 

 habited by a purely European flora. This region was perfectly prepared then, to 

 receive and establish the more or less synanthropic members of the European 

 fauna brought in by trade, but it was very little suited to provide ships destined 

 east with stowaways of North American origin. 



The economic importance of introduced animals 



It is a fact, only too well-known, that animals and plants purposely or unintention- 

 ally introduced into a foreign country often reproduce and spread very quickly 

 and soon reach an abundance far surpassing the normal conditions in their original 

 patria. As far as domesticated or otherwise useful animals are concerned this of 

 course is only to the advantage of man. More numerous, however, are the instances 

 of weeds, phytophagous insects, plant diseases, &c., which in the new continent 

 have developed to veritable pests, though many of them in their native country 

 do not cause much annoyance. 



Theoretically, the explanation of such super-normal abundance of introduced 

 species, animals as well as plants, may be found among any of the following three 

 groups of facts: — 



1. The environment, that is soil, climate, supply of food, &c., may be more 

 favourable in the new country. 



2. There may be more "space" available, the introduced species being able 

 to colonize and "ecological niche" previously not, or only thinly, inhabited by 

 indigenous species. 



3. The introduced species may have escaped from its native enemies which in 

 the original patria control its abundance. 



The first explanation seems to hold true at least in cases where a new and more 

 abundant host (plant or animal) is available in the new area. Thus the main 

 reason for the mass reproduction of the Colorado beetle [Leptinotarsa decemlineata 

 Say) was that it invaded land (in eastern North America and in Europe) where a 



