INTRODUCTION. 2/ 



shell came with timber. But Dreyssensia polymorpha 

 was by no means always confined to the Caspian and 

 Black Sea areas; it occurs abundantly in the lower 

 continental boulder-clay (see p. 230), and no doubt it 

 had at one time a much wider geographical distribution. 

 It p appears to me possible, that it was able to maintain 

 itself in certain fresh-water lakes and slow-flowing 

 rivers in Northern Europe, from which it might have 

 spread since the introduction of canals into Europe at 

 the beginning of the century. As the larva of this 

 fresh-water mussel is free-swimming, its propagation 

 is much favoured by canals. Quickly-flowing rivers 

 are fatal to its existence, since the delicate larvae are 

 swept out to sea and perish. Such an hypothesis as 

 this is strengthened by the fact of its recent discovery 

 in a sandy layer fifteen feet below the present surface 

 under the streets of London in a deposit which prob- 

 ably, as Mr. Woodward remarks (p. 8), was accumu- 

 lated in the early days of the city's existence. In 

 spite of Mr. Woodward's interesting find, and Dr. 

 Jeffreys' opinion, who always maintained that this 

 shell was indigenous to England, popular belief still 

 clings tenaciously to the introduction theory. 



Among man's intentional introductions into a new 

 country, no instance is better known than that of the 

 rabbit to Australia. Rabbits are entirely confined to 

 Europe. In their transplantation to Australia they 

 were carried to a country with a different climate and 

 among new surroundings. Yet the rabbits flourished, 

 and within comparatively few years increased to such 



