152 INTRODUCTION. 



extend themselves gradually in the country. Of these, 

 Limax agrestis, and L. variefjatiis, Helix asper-^a, IT. eel- 

 laria, II. hortensis, H. lucida, and //. pulchella, and 

 JBulimus decoUatus, B. zebra, B.fasciatus, B. virgulor 

 tus and B. luhricAis, are examples. Others strive in 

 vain to establish themselves ; the climate or some other 

 cause is fatal to them, and after some increase, they 

 diminish and become extinct. Of these, Helix nemoralis, 

 virgata, pisana, and lactea, and Bulimus acutiis, are 

 recorded as having been noticed in the United States 

 at different times ; and it is only by adopting the sup- 

 position of their subsequent extinction, that we can give 

 credit to these observations, for it is nearly certain, that 

 none of them now exist within our territory. 



Oceanic currents also aid in bringing to our shores 

 foreign species, and have been the means of introducing 

 and naturalizing them. The Gulf Stream is a promi- 

 nent example of this. This great body of water, flow- 

 ing from the Gulf of Mexico into the Atlantic, passes 

 between the Peninsula of Florida and the Island of 

 Cuba, and after turning the southern point of Florida, 

 sweeps along its eastern shore. It is sometimes driven 

 close to the northern coast of Cuba, and sometimes 

 forced much further north, according to the direction 

 and force of the wind. Various counter currents, due 

 also to the influence of the wind, diverge from the main 

 stream, among which is noticed a current which, after a 

 northerly wind has prevailed for several days, sets in a 

 south-westerly direction near to the Florida Reef. The 



