J. Se GARDNER ON BRITISH CRETACEOUS NUCULIDZ. 121 
whole of the Cretaceous series are embraced, we find types repro- 
duced at each stage which so closely resemble each other that it is 
almost impossible in some cases to formulate any distinguishing specific 
characters, and it is clear that they are in a direct line of descent. 
Yet the slight differences between the same type of Nucula or Leda 
from the Neocomian and from the Gault, or the Blackdown Beds, 
are constant and permanent modifications in some given direction, 
brought about by lapse of time or changed conditions, and once pro- 
duced never revert, and with our present system must therefore 
rank as species*. It is preferable rather to emphasize the succes- 
sive modifications than to conceal them under a common name. 
These several parallel lines of descent traverse the whole Creta- 
ceous system, and each of its stages or subdivisions contains a link 
representing a modification through lapse of time or changed physical 
conditions, in some definite direction, either towards a last repre- 
sentative species, or towards a still existing type. The group of 
“ Ovate,” after having persisted from remote periods, appears to 
have become extinct with the close of the Cretaceous, though it is 
by no means certain that we yet possess any knowledge of their 
latest representatives. Some of the most typical “ Pectinate” of 
the Cretaceous no longer survive; but the externally smooth type 
with crenulated interior margin, which appears for the first time in 
the Blackdown Beds, is the prevalent type throughout the Tertiaries 
and at the present day. 
If it were permissible to base a generalization on a single group, 
this evidence would lead us to regard the Blackdown Beds as a 
younger member of the Cretaceous system than the Gault, or than 
any other English representative of the Upper Greensand. This is 
in accord with all the other evidence, and bears out the view I have 
long held, that, taking the south-eastern counties of England and 
the N.E. of France, and thence S.E. towards Switzerland, as the 
Cretaceous centre, we shall find that as we travel away from it the 
* Tt is certainly deplorable that all these forms must, under the present system, 
have specific names conferred upon them which do not in the remotest degree 
convey the closeness or the remoteness of the relationship between them. Forms 
utterly unlike each other, and which must have been separated from their com- 
mon ancestors by scores of intervening species, and whose differentiation doubt- 
less took place in Paleozoic times, are not distinguished from each other in any 
higher degree by their nomenclature than are forms so closely related that not 
a single specific generation has intervened. It is thoroughly impossible with a 
simple binomial nomenclature to indicate the degree of relationship subsisting 
between fossil species, and it will most assuredly some day be necessary to intro- 
duce additional terms adequate to express their interrelationship. If we come 
into possession of even an approach to a perfect record of the descent of a group, 
as we may certainly hope to do, link after link being discovered, the present 
system must absolutely break down. The possible, and even probable, varia- 
tions brought about by lapse of time, changing physical conditions, and migra- 
tions during the geological period, in any family of Mollusca that has been long 
persistent, must be so endless that, as these are brought to light, it must certainly 
become impossible to classify them as species, or even subspecies; and is it not 
even now obvious that species among Brachiopods and other groups that have 
been specially studied are not the equivalents of what are ordinarily recognized 
as species among less perfectly known groups of Mollusca? 
