These herbivorous animals, which must necessarily form fat and muscle from the elements of their vegetable diet, are preyed on by the carnivorous races; and from these the carbon is again restored to the vegetable world. Sweep off from the earth the food of the herbivora, they must necessarily very soon perish, and with their dissolution, the destruction of the carnivora is certainly ensured. To illustrate this on a small scale, it may be mentioned that around the coasts of Cornwall, pilchards were formerly caught in very great abundance, in the shallow water within coves, where these fish are now but rarely seen. From the investigations of the Messrs. Couch, whose very accurate observations on the Cornish fauna have placed both father and son amongst the most eminent of British naturalists,[224] it appears that the absence of these fish is to be attributed entirely to the practice of the farmers, who cut the sea-weed from the rocks for the purpose of manuring their lands. By this they destroy all the small crustacea inhabiting these immature marine forests feeding on the algæ, and as these, the principal food of the pilchards, have perished they seek for a substitute in more favourable situations. Mr. Darwin remarks, that if the immense sea-weeds of the Southern Ocean were removed by any cause, the whole fauna of these seas would be changed. We have seen that animals and vegetables are composed principally of four elementary principles,--oxygen, hydrogen, nitrogen, and carbon. We have examined the remarkable manner in which they pass from one condition--from one kingdom of nature--into another. The animal, perishing and dwindling by decomposition into the most simple forms of matter, mingling with the atmosphere as mere gas, gradually becomes part of the growing plant, and by like changes vegetable organism progresses onward to form a portion of the animal structure.