ur PROTECTIVE RESEMBLANCES AMONG ANIMALS 49 
case the fittest will survive, and a race will be eventually 
produced adapted to the conditions in which it lives. 
We have here an illustration of the simple and effectual 
means by which animals are brought into harmony with the 
rest of nature. That slight amount of variability in every 
species, which we often look upon as something accidental or 
abnormal, or so insignificant as to be hardly worthy of notice, 
is yet the foundation of all those wonderful and harmonious 
resemblances which play such an important part in the 
economy of nature. Variation is generally very small in 
amount,! but it is all that is required, because the change in 
the external conditions to which an animal is subject is 
generally very slow and intermittent. When these changes 
have taken place too rapidly, the result has often been the 
extinction of species; but the general rule is, that climatal 
and geological changes go on slowly, and the slight but con- 
tinual variations in the colour, form, and structure of all 
animals have furnished 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 wonderful 
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—represent those few instances in 
which the process of modification 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 varia- 
tions 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 varia- 
1 Later research has shown that variation is more frequent and of greater 
amount than at first supposed. See Darwinism, chap. iii. 
E 
