Other similar gradational series of an interesting character have been noticed in the case of fresh-water fossil snail-shells. In the layers of clay and marl exposed by digging a railway cutting or a pit we may find that the successive layers represent a continuous deposit of 100,000 years or more, and we find sometimes that a form of snail-shell (not a species living to-day) occurs in the lowest stratum very different from that occurring in the highest stratum–the lowest being short and spherical, the highest elongated and of differing texture. In the intermediate layers, each 6 or 12 ins. thick and occupying perhaps altogether 30 ft. of vertical thickness, we find a graduated series of snail-shells leading almost imperceptibly from the oldest lowest form to the latest uppermost form. Such cases are known. But it is an exceptional thing to find these graduated series either spread over an area of the earth's surface, or following one another in successive strata. When they came into existence they were rapidly superseded and destroyed as a rule, and have left only one or two widely-separated examples of the intermediate forms. This we should naturally expect by analogy from what we know of the successive traces of human manufactures in the deposits on the site of some of the great cities of the ancient world which have been carefully excavated layer by layer. But still we have the important fact that here and there such gradational series have been found, and we are justified in considering a few isolated intermediate forms (which often occur connecting two greatly-differing species) as survivors of a former complete graduated series of intermediate forms, which came into existence by slow modification of an ancestral stock, and may, when the stock was widely spread over a continental area, not merely have succeeded one another in time, but actually coexisted in neighbouring regions. There are many remarkable facts bearing upon the origin of "species," the description of which fills volumes written by such men as Darwin, Wallace, Poulton, and others, and become interesting to every one who has gained a correct notion of what naturalists mean by a "species." I will cite one in order to illustrate this. The bird which we call the red grouse, or nowadays simply "grouse" (the old Scotch name for it was "muir-fowl"), is one of twenty-four birds (among the 400 species of birds which live in the British Islands), including several kinds of titmouse, the goldfinch, bullfinch, song-thrush, stonechat, jay, dipper, and others which are very closely similar to species of birds living in Continental Europe, yet show some definite and constant marks, such as small differences in the colour of a group of feathers, enabling us to distinguish the British from the Continental forms. Are these twenty-four British forms to be regarded as distinct species?