382 
DARWINISM 
CHAP. 
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 care¬ 
fully 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 unorna¬ 
mented 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 Kandberger, 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 accord¬ 
ing 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 com¬ 
plete 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, interferes with the continuity of organic development, 
and produces those puzzling discordances so generally met 
with in geological formations of marine origin. While a case 
of the kind now described affords evidence of the origin of 
species complete and conclusive, though on a necessarily very 
limited scale, the very rarity of the conditions which are essential 
to such completeness serves to explain why it is that in most 
cases the direct evidence of evolution is not to be obtained. 
