68 MIMICRY, AND OTHER PROTECTIVE 



slowly, and the slight but continual variations in the 

 colour, form, and structure of all animals, has fur- 

 nished individuals adapted to these changes, and who 

 have become the progenitors of modified races. Eapid 

 multiplication, incessant slight variation, and survival 

 of the fittest these are the laws which ever keep the 

 organic world in harmony with the inorganic, and 

 with itself. These are the laws which we believe have 

 produced all the cases of protective resemblance already 

 adduced, as well as those still more curious examples 

 we have yet to bring before our readers. 



It must always be borne in mind that the more won- 

 derful examples, in which there is not only a general 

 but a special resemblance as in the walking leaf, the 

 mossy phasma, and the leaf-winged butterfly repre- 

 sent those few instances in which the process of modi- 

 fication has been going on during an immense series 

 of generations. They all occur in the tropics, where 

 the conditions of existence are the most favourable, 

 and where climatic changes have for long periods 

 been hardly perceptible. In most of them favourable 

 variations both of colour, form, structure, and instinct 

 or habit, must have occurred to produce the perfect 

 adaptation we now behold. All these are known to 

 vary, and favourable variations when not accompanied 

 by others that were unfavourable, would certainly 

 survive. At one time a little step might be made in 

 this direction, at another time in that a change of 

 conditions might sometimes render useless that which 



O 



it had taken ages to produce great and sudden physi- 



