This interesting field of research, which the labours of that eminent observer M. Ehrenberg first made known, has since been explored by other naturalists, and in every part of the world many of the Tertiary and Secondary deposits have been found to contain microscopic organisms in profusion. At present this branch of palæontology is in its infancy, and it offers to the young student an inexhaustible and most attractive path of scientific investigation; it possesses, too, this great advantage over many others, that it can be pursued at home, and the materials for its prosecution are everywhere at hand. Unlike my explorations in the Wealden, in which a few fragments of bones, or teeth, scattered at wide intervals through the rocks, and in localities many miles apart, were often the only reward of a day's labour, here, in the quiet of my study, I may discover in a few atoms of flint, or grains of chalk, picked up by the road-side, the fossil remains of beings as interesting and extraordinary as the extinct colossal reptiles of Tilgate Forest. [Sidenote: MICROPHYTES.] The microphytes, or fossil Diatomaceæ, described in a previous chapter, (_ante_, p. 93,) were formerly classed with the organisms that now claim our attention, under the name _Infusoria_; from the belief that generally prevailed among naturalists, of their animal origin. In fact, some eminent microscopic observers, while admitting the vegetable character of Xanthidium, Micrasterias, &c. consider the Naviculæ, Ennotiæ, &c. as belonging to the animal kingdom. Thus Dr. J, W. Bailey, in a late "Memoir on the Microscopic Organisms in Various Localities of the United States," divides these bodies into three groups; viz. _Infusoria_, _Desmidieæ_, and _Diatomaceæ_; with the remark, that he has separated the two latter tribes from the Infusoria, because so many distinguished naturalists consider them decidedly to belong to the vegetable kingdom: "but," he adds, "while I believe that no positive line of separation can be drawn between certain animals and vegetables, I am yet disposed to regard the _Desmidieæ_, from the sum of all their characters, as most nearly allied to admitted vegetables; while the _Diatomaceæ_, notwithstanding Mr. Thwaites's interesting observations on their conjunction,[305] still seem to me, as they have always done, to be true animals. There is such apparent volition in their movements, such an abundance of nitrogen in the composition of their soft parts, and such resemblance between the stipitate Gomphonematæ and some of the Vorticellæ, that I should be still disposed to class them as animals, even if Ehrenberg's observations of the retractile threads and snail-like feet of some of the Naviculæ should not be confirmed."[306]