In Würtemberg, near the little village of Steinheim, are found certain strata of sand and lime, which are evidently deposits from a small lake of Tertiary times. The deposits are extremely rich in fossil shells, especially of the different species of the genus _Planorbis_. As the deposits seem to have been continuous for ages, and the fossil shells very abundant, this seemed to be an excellent opportunity to test the theory of derivation. With this end in view, they have been made the subject of exhaustive study by Hilgendorf in 1866,[34] and by Hyatt in 1880.[35] In passing from the lowest to the highest strata the species change greatly and many times, the extreme forms being so different that were it not for the intermediate forms they would be called not only different species but different genera. And yet the gradations are so insensible that the whole series is nothing less than a demonstration, in this case at least, of origin of species by derivation with modifications. The accompanying plate of successive forms (Fig. 70), which we take from Prof. Hyatt’s admirable memoir, will show this better than any mere verbal explanation. It will be observed that, commencing with four slight varieties--probably sexually isolated varieties--of one species, each series shows a gradual transformation as we go upward in the strata--i. e., onward in time. Series I branches into three sub-series, in two of which the change of form is extreme. Series IV is remarkable for great increase in size as well as change in form. In the plate we give only selected stages, but in the fuller plates of the memoir, and still more in the shells themselves, the subtilest gradations are found. [Illustration: FIG. 70.--Transformations of Planorbis (after Hyatt). _Series IV._ 1, Pl. levis: Undorf. 2, Pl. Steinheimensis; 3, tenuis-Steinheimensis; 4, tenuis; 5, discoideus; 6, trochiformis-discoideus; 7, trochiformis: Steinheim. _Series III._ 8, Pl. levis: Undorf. 9, Pl. oxystomus; 10, supremus; 11, supremus var. turrita: Steinheim. _Series II._ 12, Pl. levis: Undorf. 13, Pl. crescens-parvus; 14, 15, crescens: Steinheim. _Series I._ _Sub-series 3._ 16, Pl. levis: Undorf. 17, Pl. minutus-levis; 18, minutus; 19, 20, triquetrus: Steinheim. _Sub-series 2._ 21, Pl. minutus; 22, 23, denudatus-minutus; 24, denudatus var. distortus: Steinheim. _Sub-series 1._ 25, Pl. costatus-minutus; 26, costatus; 27, 28, costatus var--: Steinheim. The specimens from Undorf all belong to an older Tertiary period than that at Steinheim. ]