And it is well to reflect that the only alternative supposition is, that these numerous successive and coexistent forms of insignificant animals, the differences of which require careful study for their discrimination, have been separately and independently fabricated, and put into the localities in which we find them. By whatever verbal fog the question at issue may be hidden, this is the real nature of the dilemma presented to us not only by the crayfish, but by every animal and by every plant; from man to the humblest animalcule; from the spreading beech and towering pine to the _Micrococci_ which lie at the limit of microscopic visibility. {347} NOTES. NOTE I., CHAPTER I., p. 17. THE CHEMICAL COMPOSITION OF THE EXOSKELETON. The harder parts of the exoskeleton of the crayfish contain rather more than half their weight of calcareous salts. Of these nearly seven-eighths consist of carbonate of lime, the rest being phosphate of lime. The animal matter consists for the most part of a peculiar substance termed _Chitin_, which enters into the composition of the hard parts not only of the _Arthropoda_ in general but of many other invertebrated animals. Chitin is not dissolved even by hot caustic alkalies, whence the use of solutions of caustic potash and soda in cleaning the skeletons of crayfishes. It is soluble in cold concentrated hydrochloric acid without change, and may be precipitated from its solution by the addition of water. Chitin contains nitrogen, and according to the latest investigations (Ledderhose, “Ueber Chitin und seine Spaltungs-produkte:” Zeitschrift für Physiologische Chemie, II. 1879) its composition is represented by the formula C_{15}H_{26}N_{2}O_{10} . NOTE II., CHAPTER I., p. 29. THE CRAB’S EYES, OR GASTROLITHS.