INTRODUCTION. 
XXVll 
cold, all are perpetual agents in this one work. If we 
seek for the accessory circumstances most favourable to 
the rapid and healthy growth of ferns, and refer for the 
information to Natoe herself, we shall generally find 
them in protection from the sun’s rays, in the uniformity 
and excess of atmospheric humidity, in the absence of 
extremes of heat and cold, in the gradual transition from 
one to the other, when these extremes do occur, and, 
finally, in that perfect stillness of the atmosphere which is 
rarely realized in Nature, except in caves, fissures of rocks, 
wells, and a few similar situations : the opposites of all 
these are the agents of decay and destruction, — the excess 
of atmospheric aridity, sudden alterations in the tempera- 
ture, as in the frosts of spring, excessive heat, high and 
boisterous winds. Were not this law of destruction in per- 
petual operation, as well as the law of renovation — were 
they not invariably linked as it were hand in hand, the 
surface of the earth would become in one extreme a desert, 
untenanted by living things, in the other a self-destructive 
crowd. 
Returning to the phial, and therefore to all closed vessels 
or buildings, we cannot fail to perceive, that while all the 
agents of life — all the vivifying principles — are allowed 
the fullest scope for their operations ; the destructive ones 
are in a greater or less degree excluded Nature is still at 
work ; no particle of the benefit results from human skill ; 
we add no gases to those around us in order to make the 
air more nourishing ; we subtract none to make it more 
pure. Atmospheric humidity is one of the most important 
