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.
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.
This case is striking, partly because it is a very favorable one, but mainly because it has been so carefully studied. There can be no doubt that equally careful study would reveal the same transition in many other cases. Nor are such transitions confined to the lower forms of life, though they are probably more abundant there. According to Cope, the nicest gradations may be traced between some of the extinct mammalian species so abundant in the Tertiary deposits of the West—especially between the species of the extinct generalized family of Oredontidæ[36] The same is probably true of the many extinct species of the horse family.
It is interesting to observe that the details of the process of change in the forms of Planorbis are in accord with Dr. Romanes’s views. The change does not seem to have been uniform but somewhat paroxysmal. The forms seem to remain stable for a long time, and then a few break into several different forms, while the more rigid die out. It is as if cross-breeding had kept the type true, but at the same time increased its tendency to variation, until finally one or more varieties became sexually isolated and thus formed new species.