interesting evidence
of the origin of new forms through descent with modification is
that of the genus Vivipara or Paludina, which occurs in
prodigious abundance throughout the whole series of freshwater
strata. We shall not, of course, attempt in this place to enter
into any details concerning the forty distinct _forms_ of this
genus (Dr. Neumayr very properly hesitates to call them all
_species_), which are named and described in this monograph,
and between which, as the authors show, so many connecting
links, clearly illustrating the derivation of the newer from the
older types, have been detected. On the minds of those who
carefully examine the admirably engraved figures given in the
plates accompanying this valuable memoir, or still better, the
very large series of specimens from among which the subjects of
these figures are selected, and which are now in the museum of
the Reichsanstalt of Vienna, but little doubt will, we suspect,
remain that the authors have fully made out their case, and have
demonstrated that, beyond all controversy, the series with
highly complicated ornamentation were variously derived by
descent--the lines of which are in most cases perfectly clear
and obvious--from the simple and unornamented Vivipara
achatinoides of the Congerien-Schichten (the lower division of
the series of strata). It is interesting to notice that a large
portion of these unquestionably derived forms depart so widely
from the type of the genus Vivipara, that they have been
separated on so high an authority as that of Sandberger, as a
new genus, under the name of Tulotoma. And hence we are led to
the conclusion that a vast number of forms, certainly exhibiting
specific distinctions, and according to some naturalists,
differences even entitled to be regarded of generic value, have
all a common ancestry."

It is, as Professor Judd remarks, owing to the exceptionally favourable
circumstances of a long-continued and unbroken series of deposits being
formed under physical conditions either identical or very slowly
changing, that we owe so complete a record of the process of organic
change. Usually, some disturbing elements, such as a sudden change of
physical conditions, or the immigration of new sets of forms from other
areas and the consequent retreat or partial extinction of the older
fauna, interfer