INTRODUCTION. 25 



similar range naturally. The latter he calls " palin- 

 cosmic " species (a, p. 422). Many so-called ceno- 

 cosrnic ants are believed by Dr. von Ihering to be 

 palincosmic. We are altogether too apt to regard 

 cosmopolitan as synonymous with introduced, and we 

 should hesitate before concluding that because one of 

 .our common European species occurs in Australia or 

 South America, it must have been transported there 

 recently by human agency. Some of our widely- 

 distributed forms are probably of very great antiquity, 

 and may have spread to distant lands in early Ter- 

 tiary times, when a different state of the geographical 

 conditions enabled them to do so. 



I cannot quote a more appropriate instance 

 than the molluscan fauna of Madeira. No less 

 than thirteen of the Madeiran snails are looked 

 upon as having been introduced from Europe by 

 human agency, on the sole evidence that these 

 happen to be common European species. Yet the 

 correctness of this supposition must be questioned 

 in face of the interesting observation made by 

 Darwin (p. 357), "that Madeira and the ad- 

 joining islet of Porto Santo possess many distinct 

 but representative species of land-shells, some of 

 which live in crevices of stone ; and although large 

 quantities of stone are annually transported from 

 Porto Santo to Madeira, yet this latter island has not 

 become colonised by the Porto Santo species. Never- 

 theless, both islands have been colonised by European 

 land -shells, which no doubt had some advantage 



