The Naturalist in Nicaragua | Page 5

change by explaining them as the result of cosmical causes, and Croll's theory of the increase of the eccentricity of the earth's orbit was widely received among them. Belt, on the other hand, held that the cold was due to an increase in the obliquity of the ecliptic. But these astronomical explanations have not met with much acceptance by physicists; and so chemists have been turned to by some geologists for support of the hypothesis of the variation in the amount of carbon dioxide in the air, or of other alterations in the atmosphere, while others have gone back to the idea of geographical changes. That considerable oscillations of the relative levels of land and sea took place during the Ice Age has been now clearly established, and the general result of the investigations favours Belt's opinion that the land during part of that period 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 evolution 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 Species" in 1859, Belt had become an ardent evolutionist, 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 protective resemblances, 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 resemblance 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 vertebrate animal, nearly always bears the double meaning, as in the green tree frog, where the colouration is protective so far as it provides concealment from snakes, which are particularly fond of these frogs, and aggressive in that it allows flies and other insects to approach without suspicion.
There may be either General Resemblance to surrounding objects or Special Resemblance to definite objects. The plain sandy colour of desert animals, the snow white of the inhabitants of the arctic regions, the inconspicuous hues of nocturnal animals, the stripes of the tiger and the zebra, the spots of the leopard and the giraffe have all a cryptic effect which at a very short distance renders the creatures invisible amid their natural surroundings. Nor is it necessary in order to attain this invisibility that the colouring should be really dull and plain. It all depends upon the habitat. Mr. Wallace has described "a South American goatsucker which rests in the bright sunshine on little bare rocky islets in the upper Rio Negro where its unusually light colours so closely resemble those of the rock and sand that it can scarcely be detected till trodden upon." A little observation will supply large numbers of instances of such protective colouration.
It is, however, in the insect world that this principle of adaptation of animals to their environment is most fully and strikingly developed. "There are thousands of species of insects," says Mr. Wallace again, "which rest during the day clinging to the bark of dead or fallen trees; and the greater portion of these are delicately mottled with grey and brown tints, which though symmetrically disposed and infinitely varied, yet blend so completely with the usual colours of the bark, that at two or three feet distance they are quite undistinguishable."
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