26 HISTORY OF THE EUROPEAN FAUNA, 



over the indigenous species." Darwin, therefore, 

 meets the evident anomaly by suggesting that the 

 European species are supposed to possess some 

 advantages as colonisers. But the true explanation 

 appears to me to lie in the supposition that the 

 European land-shells found in the Madeiran Islands 

 are all, or for the greater part, ancient forms which 

 survived both there and on the continent, whilst the 

 remainder of the forms inhabiting these islands are 

 either such as are now extinct in Europe, or have 

 become modified since their arrival there from the 

 continent at a time when extensive land-connections 

 allowed a free migration by land. 



The theory of accidental introductions is an ex- 

 tremely popular one. It allows free scope to a host 

 of speculations, and once the idea has taken firm 

 root that a certain species is introduced, especially 

 among the class of naturalists who by way of ex- 

 periment are wont to create new centres of dispersion 

 in their own neighbourhood, evidence to the contrary 

 must be of the most convincing nature to shake the 

 popular belief. Thus, it is almost regarded as an 

 established fact by conchologists and others, that the 

 fresh-water mussel (Dreyssensiapolymorpka) was intro- 

 duced into England at the beginning of this century. 

 Though it has been proved that this species is quite 

 unable to live in pure sea-water, yet the view that it 

 has been carried from the Black Sea ports to this 

 country attached to the bottom of ships is maintained 

 by many, whilst others incline to the theory that the 



