Of the recent species, 575 inhabit the Tropics. ------ 350 ----- Temperate zones. ------ 75 ----- Cold regions. The above statistical view was published six years ago; but the great activity of research that has since prevailed, has largely augmented the known number both of fossil and recent forms. M. D'Orbigny's recent Tables[346] give for the Jurassic or Oolite 10 genera; Cretaceous, 38 genera; Tertiary, 60 genera; but this estimate must be far too low. [346] Cours Elémentaire de Paléontologie. * * * * * I have thus endeavoured to convey a general idea of the highly important results obtained by the microscopical investigation of the minute organisms that enter so largely into the composition of many of the fossiliferous deposits. Without the aid of the most perfect optical instruments which modern science and art have produced, even the existence of many of these structures could not have been demonstrated; and we cannot doubt, that were the powers of the microscope increased, the fossil remains of beings still more minute would be detected; and that rocks and strata which now appear to consist of amorphous particles of lime, of silex, and of iron, would prove to be the aggregated skeletons of animals, yet more infinitesimal than those which have formed the subject of our contemplations. How strikingly illustrative are these phenomena of the profound remark of the illustrious Galileo--"_La nature fait beaucoup avec peu, et ses opérations sont toutes également merveilleuses._" INSTRUCTIONS FOR THE MICROSCOPICAL EXAMINATION OF CHALK, FLINT, AND OTHER ROCKS. [Sidenote: MICROSCOPICAL EXAMINATION OF CHALK.]