INTRODUCTION. 13 



deeper home, where, as its numbers increase, there 

 must be a tendency to spread. We have in these 

 cases, therefore, both a periodic and an ordinary 

 movement of migration. 



Now, in studying the composition of a fauna, and 

 especially its origin, it is of the utmost importance to 

 be able to determine approximately the percentage of 

 accidental arrivals and of the ordinary migrants 

 that is to say, of those which have reached the 

 country owing to accidental distribution, and of 

 animals which have adopted the usual course of 

 migration. It is of all the more import to review 

 this subject of accidental, or, as Darwin called it, " the 

 occasional means" of distribution, as both he and 

 Dr. Wallace have, I venture to think, somewhat over- 

 estimated its significance. No one doubts that acci- 

 dental transportal takes place, but the question is 

 whether the accidentally transported animals arrive 

 living and reach a spot where suitable food is pro- 

 curable, and whether they are able to propagate their 

 own species in the new locality. For it must be clear 

 to anybody that the accidental transportal of a beetle 

 or of a snail to a new country cannot affect its fauna 

 or add one permanent member to it unless all these 

 conditions are fulfilled. As a matter of fact, only 

 exceedingly few instances are on record of man 

 having witnessed, for example, the accidental 

 transportal across the sea to an island of a live 

 animal. 



To mention an example, Colonel Feilden informs 



