30 FRIENDS WORTH KNOWING. 
The astonishing vitality of the snails in every stage of 
existence favors the theory that they endure such acciden- 
tal means of travel and thrive at the end of it. Professor 
Morse records that he has seen certain species frozen in 
solid blocks of ice, and afterward regain their activity; and 
enduring an equal extreme of heat, where the sun’s rays 
crisped the leaves for weeks together, without any bad ef- 
fect. They have been shut up for years in pill-boxes, glued 
for years (seven years in one case, Dr. Newcomb, of Cornell 
University, told me) to tablets in museums, and yet a trifle 
of moisture has been sufficient to resuscitate them. They 
survive so well being buried in the ballast of ships that at 
every seaport, almost, you may find species imported in that 
way, which came to life when the ballast was dumped at 
the time of unloading. That birds occasionally carry them 
about is well verified. 
Such are some of the methods of dispersion. Yet stu- 
dents are obliged to confess that the causes of the present 
puzzling geographical distribution of land shells are so com- 
plex that we can hardly hope to determine them with much 
exactness. 
As to the longevity of snails, little is known; but some 
individuals no doubt attain great age. Some species of 
cylindrella have a habit of deserting the point of the spire 
