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.