ristics
appear in the formation. Those who wish more information about embryology
can find it in Heinrich Rathke's "Entwicklungsgeschichte der Wirbelthiere"
("History of the Development of Vertebrates"), edited by A. Koelliker,
Leipzig, Engelmann, 1861; and those who wish to inform themselves as to the
influence of the ontogenetic results of the solution of the phylogenetic
problems, will find, besides the before-mentioned work of Wigand, rich and
clearly elaborated material in the publication of Wilhelm His--"Unsere
Koerperform und das physiologische Problem ihrer Entstehung, Briefe an einen
befreundeten Naturforscher" ("The Form of our Body and the Physiological
Problem of its Origin; Letters to an Associate Scientist"), Leipzig, Vogel,
1875. The latter writer, although he advocates the descent theory, rejects
the hasty assertions of Haeckel with direct and convincing arguments.

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 t