GEOGRAPHICAL DISTlllBUTlOX. 1'21 



111 addition to the causes mentioned before, as exer- 

 cising a general influence, there are others operating 

 in a more limited sphere, and acting only at particular 

 times, the effect of which, nevertheless, is for the time 

 being very important, and they are therefore deserving 

 of remark. They may be denominated occasional or 

 accidental causes. 



Sadden and extreme Vicissitudes of Temperature. 

 The terrestrial mollusks resist changes of temperature 

 better than many others by the defences they construct 

 over the mouth of the shell, as well as by retiring into 

 protected situations. There is, however, a limit to their 

 powers of resistance, and sudden and extreme depres- 

 sions of temperature, particularly in those parts of the 

 coimtry where such changes are uncommon, are very 

 fatal to them. In the winter of 1834 and 1835 a sud- 

 den and severe frost continuing for several hours, de- 

 stroyed all the orange trees in the Territory of Florida, 

 causing a most serious loss to the inhabitants. At the 

 same time, the tide being at ebb, the marine mollusks 

 living above low water mark were nearly exterminated, 

 and immense numbers of the terrestrial species were 

 killed. We have noticed the surface of small islands on 

 the coast of Florida literally covered with the bleached 

 remains of Hehx and Hehcina that were destroyed at 

 that time. It must have required several years to re- 

 store the species to the same position in point of numbers 

 which they held before that occurrence. Cold, not very 

 extreme in degree, but continued for a longer period than 

 usual, gradually produces results of a similar character. 



