Such, then, are a few of the outstanding problems that confront the soil protozoologist; but he must always remember that the organisms he studies are but a small fraction of the total, and that any influence affecting one part of the complex will be reflected in another. As Prof. Arthur Thomson said in his Gifford Lectures, “No creature lives or dies to itself, there is no insulation. Long nutritive chains often bind a series of organisms together in the very fundamental relation that one kind eats the others.” Such nutritive chains obtain in the soil as markedly as in other haunts of living creatures. SELECTED BIBLIOGRAPHY. * _Papers giving extensive bibliographies._ [1] Cunningham, A., Journ. Agric. Sci., 1915, vol. xvii., p. 49. [2] Cunningham, A., and Löhnis, F., Centrlb. f. Bakt. Abt. II., 1914, vol. xxxix., p. 596. [3] Cutler, D. W., Journ. Agric. Sci., 1919, vol. ix., p. 430. [4] Cutler, D. W., Journ. Agric. Sci., 1920, vol. x., p. 136. [5] Cutler, D. W., Ann. App. Biol., 1923, vol. x., p. 137. [6] Cutler, D. W., and Crump, Ann. App. Biol., 1920, vol. vii., p. 11. [7] * Cutler, D. W., Crump and Sandon, Phil. Trans. Roy. Soc. B., 1922, vol. ccxi., p. 317. [8] Delf, E. M., New Phytologist, 1915, vol. xiv., p. 63. [9] Dobell, C. C., Arch. f. Protisenk., 1911, vol. xxiii., p. 269. [10] * Goodey, T., Roy. Soc. Proc. B., 1916, vol. lxxxix., p. 279. [11] Goodey, T., Roy. Soc. Proc. B., 1913, vol. lxxxvi, p. 427. [12] Hartmann, M., and Nägler, K., Sitz-Ber. Gesellsch. Naturf. Freunde, 1908, Berlin, No. 4. [13] Koch, G. P., Journ. Agric. Res., 1916, vol. li., p. 477. [14] Kofoid, C. A., Bull. Illinois State Lab. Nat. Hist., 1903 and 1908. [15] * Kopeloff, N., Lint, H. C., and Coleman, D. A., Centrlb. f. Bakt. Abt. II., 1916, vol. xlvi., p. 28. [16] Kopeloff, N., Lint, H. C., and Coleman, D. A., Centrlb. f. Bakt. Abt. II., 1916, vol. xlv., p. 230. [17] Lipman, J. G., Blair, A. W., Owen, L. L., and McLean, H. C., N.J. Agric. Exp. Sta. 1912, Bull., No. 248. [18] Martin, C. H. and Lewin, K. R., Journ. Agric. Soc., 1915, vol. vii., p. 106. [19] Martin, C. H., Roy. Soc. Proc. B., 1912, vol. lxxxv., p. 393. [20] * Nasir, S. M., Ann. App. Biol., 1923, vol. x., p. 122. [21] Russell, E. J., Roy. Soc. Proc. B., 1915, vol. lxxxix., p. 76. [22] * Russell, E. J., “Soil Conditions and Plant Growth,” 1921, 4th ed. [23] * Sherman, J. M., Journ. Bact., 1916, vol. i., p. 35, and vol. ii., p. 165. [24] Truffaut, G., and Bezssonoff, H., Compt. Rend. Acad. Sci., 1920, vol. clxx., p. 1278. [25] West, W., and West, G. S., Journ. Linn. Soc. Bot., 1912, vol. xl., p. 395. CHAPTER VI. ALGÆ. I. GENERAL AND HISTORICAL INTRODUCTION.