GEOGRAPHICAL DISTRIBUTION. 127 
In 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. 
Sudden 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 
country where such changes are uncommon, are very 
fatal to them. In the winter of 1884 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 Helix and Helicina 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. 
