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XLLl EEFEEENCE TO THE ORIGIN OF SPECIES. 



429 



If, for example, we compare the landshells of the Wilt- 

 shire drift, near Salisbmy, of the age of the Mammoth, with 

 those of Essex before mentioned, places twice as far apart 

 as are Madeira and Porto Santo, they exhibit no difference 

 whatever in the species of fossil landshells. From this. fact 

 we may infer that although the British area has been partially 

 submerged since the commencement of the Glacial period, 

 yet its normal state has been a continental one. 



Mode in which an oceanic island might become peopled with 

 landshells. — The reader may well ask, if Madeira and Porto 

 Santo have made so little progress in interchanging their 

 respective species of landshells in the course of that vast 

 lapse of ages which has occurred since the Newer Pliocene 

 period, how could any of the Atlantic archipelagos ever have 

 become peopled by migration from Europe or Africa ? The 

 enigma is certainly perplexing, and we must assume that the 

 arrival of landshells, as waifs and strays from a continent, is 

 an exceedingly rare event. It has been suggested that birds 



may transport across the sea the eggs of these moUusks in 



mud attached to their feet. But if so, why have the birds 

 which fly freely across the channel, only 30 miles wide, be- 

 tween Madeira and Porto Santo, allowed the fauna of these 

 two islands to remain so distinct ? or why have those birds 

 which arrive every year from the continent in the Atlantic 

 islands introduced so few landshells ? Hitherto the naturalist 

 has not witnessed the arrival of a new continental Helix on 

 any remote oceanic island, except by the aid of man ; and to 

 those who are unwilling to abandon in despair all hope of 

 solving the problem, it is 



the case. How 



m 



from 



it is satisfactory that such should be 



inexplicable would be the dearth of land 



the Atlantic islands if some members of 



seen occasionally to swim across the ocean 



If hereafter we should 



mode in which 



air- 



moUusks can sometimes 



r 



70 may be sure that the o 



be few and far between, so that a continental species when 



time 



to one or two new races, before other representatives of the 



