Chapter II 



THE HUMAN TRANSPORT OF ANIMALS ACROSS THE 

 NORTHERN ATLANTIC 



The five criteria of an introduced species 



JVlan has purposely carried several animal species, and not only domesticated 

 ones, across the Atlantic in either direction, and many of them have become per- 

 manently established (instances, vide above, p. i6). By far more numerous are 

 the animal and plant species which were carried over unintentionally by all kinds 

 of ships. Of course only a small fraction of them were able to find and colonize 

 suitable ground in the new continent and the successful event, with very few ex- 

 ceptions, was not apparent until long afterwards when it was impossible to re- 

 construct the procedure accurately. Consequently, in trying to sort out the foreign 

 elements of recent introduction from the indigenous fauna of a continent, we are 

 usually restricted to second hand evidence: certain features of distribution, abund- 

 ance, ecology, &c., shown by a species in the country where it is believed to have 

 been introduced. There are reasons to presume that such characteristics of an 

 immigrant into North America from Europe, or vice versa, as a rule have not been 

 completely blotted out during the comparatively short period of, at most, 460 years. 

 The foremost criteria of an introduced species are: 



I. THE HISTORICAL CRITERION. This includes all cases in which we are able 

 to trace by historical evidence, if not the very landing of a species, which is a 

 rare exception, at least parts of the following expansion. It is true that the period 

 open to scientific survey is too short to control the normal changes of a species' 

 area, but the recent immigrants often show a surprising power of rapid dispersal 

 during the interval between first establishment and balanced distribution. Thus 

 the introduction, temporary extermination, re-introduction, and final victorious 

 progress of the Colorado-Beetle {Leptinotarsa decemlineata Say) in Europe (fig. 3), 

 as well as the history of the Gypsy Moth {Lymantria dispar L.) in North America, 

 were followed in detail from year to year. 



Similar instances are provided by Passer domesticus L. (the English sparrow) and 

 Sturnus sturnus L. (the Starling; fig. 4), which are known to be introduced into North 

 America, as well as the Muskrat {Fiber or Ondatra zibethicus L.) in Europe (fig. 5), 

 and by Pieris rapae L. (the Cabbage Butterfly), Phytonomus posticus Gyll. (the 



