Introduction XV 
stood much higher than now over the northern regions of 
Europe and North America. It would, however, lead us 
too far away from the present book to enter into even a 
cursory examination of his views upon the glacial period, 
and those readers who desire to pursue the matter will find 
assistance for doing so in the bibliography at the end of 
this Introduction. . 
Of more immediate interest to us are the “ observations 
on animals and plants in reference to the theory of evolu- 
tion of living forms ”’ which the title-page announces as a 
part of the narrative, and which indeed form the main 
portion of the work. Upon the publication of Darwin’s 
Origin of Spectes in 1859, Belt had become an ardent evolu- 
tionist, and was henceforth always on the look-out for 
facts in support of the theories which had breathed such 
new life into biological studies. In Nicaragua he devoted 
special attention to those wonderful protectiveresemblances, 
especially among insects, which Bates had explained by 
his theory of “‘ Mimicry; ”’ and as the subject crops up again 
and again in this book, the non-scientific reader will find it 
helpful to have before him an outline of the expanded 
and completed theory—though he should be warned that 
some writers have been too much inclined to attribute 
to “‘mimicry’”’ any accidental resemblance between two 
species. How far such accidental resemblances may be 
carried is probably well illustrated by the bee, the spider, 
and the fly orchis of our own downs and copses. 
““Mimicry ” proper is often confused with ‘“‘ protective 
resemblance,” and it will be advisable to begin with the 
consideration of the latter. 
Concealment, while useful at times to all animals, is 
absolutely essential to some; and it is wonderful in what 
different ways it is attained. In cases of ‘‘ cryptic resem- 
blance to surroundings ”’ the shape, colouration, or markings 
are such as to conceal an animal by rendering it difficult 
to distinguish from its immediate environment. In most 
cases the effect is protective ; but in snakes, spiders, mantids, 
and other preying animals it is termed aggressive, since it 
enables these animals to stalk their prey undetected. It 
is probable that this power, when possessed by a verte- 
brate animal, nearly always bears the double meaning, as 
in the green tree frog, where the colouration is protective 
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