Thus embryology, having from the simple fact of an origin of single plants and animals through descent at least confirmed the idea of the _possibility_ of an origin also of species through development, forsakes us in the {82} inquiry as to the _reality_ of such a genealogy of development, and refers us to other sciences. Such a science, from which we certainly are entitled to expect a decided answer, is _geology_. For if the evolution theory is right, those periods of the history of our globe in which new species originated--namely, the periods of geology--_must_ show us also the _forms of transition_ between the different species. And, indeed, geology gives us an answer; but it reads contradictorily: It says yes, and it says no. Geology does show us forms of transition, and, indeed, most frequently in the lower classes of animals. Who that has once studied petrifactions, does not know the mass of forms of the terebratulae, the belemnites, and the ammonites, in the Jura formation? Wuertemberger has brought light into the perplexing division of species of the ammonites by simply showing their temporary and systematic transitions into one another. In the fresh water chalk formation of Steinheim, near Heidenheim, in Wuertemberg, scientists have found, on the same place, in an uninterrupted series of strata, the snail valvata or paludina multiformis in all imaginable transitions--from the flat winding, showing the form of a chess-board, up to the sharp form of a tower. And it was not, as Hilgendorf thought, in a series which can be traced in the strata according to time, but, as Sandberger says, in quite a varied mixture, yet in all imaginable modifications. But even among the higher and the highest classes of animals, we can trace the transitions. The flying sauria, if not in their organs of flying, which remind us more of the bat, at least in head, neck, and toes, are closely connected with the {83} birds--the oldest birds of the Jura and chalk formations, with their tail-spines similar to the reptilia and their teeth in the beak to the sauria. The tertiary formations especially show the primitive history of many vertebrates in very instructive forms of transitions--which, for instance, Ruetimeyer, a scientist who is very cautious in his conclusions, very distinctly traced to the horse, to the ruminating animals, and lately also to the turtles. Still more in detail, W. Kowalewsky has lately shown us the primitive history of the horse, and Leidy and Marsh have further completed it by the addition of American forms, the former having at the same time described the forms which have led to the tapir.