194 INTRODUCTION. 



together. In clouclj weatlier, when the atmosphere is 

 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. 



We 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 sIoav 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 be 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 prohfic generative capacity, their insensibility 

 to pain, their extreme tenacity of hfe, 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 are exposed are greater or less. Thus, in 



