A similar series of shells has been brought to light in Württemberg in which the variations pass through wider limits, so that not only different species may be observed, but different genera connected by almost insensible gradations. These transformations are found in a little flattened pond-shell similar to the planorbis, which is so common at the present time. [Illustration: Fig. 104.--Planorbis Shells from Steinheim. (After Hyatt.)] Fig. 104 shows some of these transformations, the finer gradations being omitted. The shells from these two sources bear directly upon the question of whether or not species have held rigidly to their original form. After this kind of revelation in reference to lower animals, we turn with awakened interest to the fossil bones of the higher animals. Evolution of the Horse.--When we take into account the way in which fossils have been produced we see clearly that it is the hard parts, such as the shells and the bones, that will be preserved, while the soft parts of animals will disappear. Is it not possible that we may find the fossil bones of higher animals arranged in chronological order and in sufficient number to supplement the testimony of the shells? There has been preserved in the rocks of our Western States a very complete history of the evolution of the horse family, written, as it were, on tablets of stone, and extending over a period of more than two million years, as the geologists estimate time. Geologists can, of course, measure the thickness of rocks and form some estimate of the rate at which they were deposited by observing the character of the material and comparing the formation with similar water deposits of the present time. Near the surface, in the deposits of the quarternary period, are found remains of the immediate ancestors of the horse, which are recognized as belonging to the same genus, Equus, but to a different species; thence, back to the lowest beds of the tertiary period we come upon the successive ancestral forms, embracing several distinct genera and exhibiting an interesting series of transformations. If in this way we go into the past a half-million years, we find the ancestors of the horse reduced in size and with three toes each on the fore and hind feet. The living horse now has only a single toe on each foot, but it has small splint-like bones that represent the rudiments of two more. If we go back a million years, we find three toes and the rudiments of a fourth; and going back two million years, we find four fully developed toes, and bones in the feet to support them. It is believed that in still older rocks a five-toed form will be discovered, which was the parent of the four-toed form.