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. Rapid 
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 
it had taken ages to produce—great and sudden physi- 
