Races which once formed vast and lofty forests are now represented by a few lowly herbs; and it is difficult to recognize in the tiny _Selaginella_ of our moors the representative of the gigantic Club-mosses of Carboniferous days. But certain plants still living retain to a great extent the features of their ancestors of the ancient rocks. One of the most interesting of these is the Maidenhair Tree (_Ginkgo biloba_), well known as a sacred tree in the East, and apparently preserved to us through the last few thousand years owing to this custom, as it does not seem to exist now in a wild state. The genus _Ginkgo_ runs back to the beginning of the Mesozoic Period, and its near relatives go back much farther still to the Devonian; the group to which it belongs, _Ginkgoaceæ_ (probably descended from the _Cordaiteæ_), attains its maximum in the Jurassic, the “Age of Reptiles,” and the existing species or its near relatives saw the Earth teeming with fantastic Saurians, including huge brutes, longer than the greatest whale, which browsed on trees or devoured creatures scarcely less terrible than themselves, while others of different form, occupied the sea, and others again of nightmare appearance dashed bat-like through the air. This solitary representative of a great and ancient race is of quite peculiar interest in that it is the highest plant in which is preserved the primitive feature of fertilization by the medium of water, the male cell being endowed with the power of motion, and reaching the egg-cell by means of swimming.