19_J. INTRODUCTION. 
together. In cloudy weather, when the atmosphere ia 
charged with moisture, and during light showers, all the 
species come forth in the day time ; but on a change of 
weather, immediately return again, and during rains 
remain in their retreats. Long continued or excessive 
rains, however, inundate their hiding places, drive them 
out, and force them to resort to trees. 
\\ e have seen, in a preceding part of this work, how 
numerous are the agencies which are continually tending 
to destroy the lives of individuals, and to exterminate 
whole species. Being all of them slow in their motions, 
without means of escape from enemies, destitute of in- 
struments .of offence or of defence, and some of them 
unprovided with a covering, it would seem as if their 
existence must he very precarious, and that they must 
be easy victims to the unfavorable circumstances around 
them. Such would be the case undoubtedly, and these 
causes would interfere with the diffusion of species and 
derange their distribution in a greater degree than they 
actually do, if there were not counteracting properties 
in the animals themselves which modify and limit the 
destructive tendency. These conservative properties 
are, their prolific generative capacity, their insensibility 
to pain, their extreme tenacity of life, and their extra- 
ordinary power of reproducing important organs which 
have been cut off or destroyed by accident. 
The number of eggs produced varies in the genera 
and species in the same proportion as the dangers to 
which they arc exposed are greater or less. Thus, in 
