192 SUPPLEMENT TO THE CRAG MOLLUSCA. 
life will be acquired, yet every increase of our knowledge of natural history must bring us. 
nearer to this state of things, and proportionately augment our difficulties im the way of 
specific separation. In the present state of our knowledge, therefore, it seems to me more 
philosophical, and likely to be more advantageous in working out the history of the past 
changes of land and water on the globe, if the identification of species be not strained ;. 
and that, wherever a form presents differences from any other, though they be but slight, 
and those differences are fairly maintained in a group of individuals without intermediate 
forms occurring coevally in the same geographical area, such form should be regarded as 
a distinct species ; in my ‘Monograph of the Eocene Bivalves’ I have expressed my 
views on that point more fully. Another reason for not undervaluing even shght 
differences by which many of the Crag Mollusca are separable from their living analogues, 
and so reducing them to the inferior importance supposed to be possessed by the term 
“variety,” exists in the discordance between the evidence presented by the Molluscan 
fauna when thus reduced, and that presented by the other organisms of the Crag 
period. Thus the evidence of the Hztomostraca, the Foraminifera, the Polyzoa, the 
Polyparia, the Cirripedia, and the Echinodermata of the Crag (all of them studied and 
described by independent authors) has quite a different bearing from that of the 
Molluscan fauna, when this last is reduced in the way it has been by the author of the 
‘ British Conchology.’ 
In the case of the Hntomostraca the proportion of species not recognised as living is 
as 13 to 5, of the Moraminifera as 47 to 53, of the Polyzoa as 65 to 30, of the 
Polyparia 3 to 1, of the Cirripedia as 4 to 6, and of the Behinodermata as 13 
(iO Be 
Now, although the researches which have been carried on among the living species of 
these groups of organisms may not have been so extensive as those carried on among the 
Mollusca, and although we may thus be better acquainted with the living forms of the 
latter, still, after making very large allowances on this account, we are left with great 
discrepancies between the evidence afforded by the percentage of forms not known living 
among the Mollusca, and those among the other groups. These discrepancies are so 
striking as to suggest caution in accepting the process by which the list of Crag species 
has been pared down, and so many species eliminated from it in the lists which accompany 
Mr. Prestwich’s Crag papers. 
The authors of the ‘ British Mollusca,’ like myself, regard the Molluscan fauna of the 
Coralline Crag as having its affinities chiefly with that of the Mediterranean; but the 
1 A table of the proportions borne of living to extinct forms among these various groups of organism 
” 
will be found at p. 134 of Mr. Prestwich’s paper on the “Cor. Crag.” This agrees substantially with the 
analysis given by me in the text. The number of species of Coralline Crag Mollusca, however, according 
to the list by Mr. Jeffreys, which accompanies the paper of Mr. Prestwich, is 316, of which he considers 
264 to be living, and 52 extinct; thus giving a percentage of 84 recent and 16 extinct. ‘ Quart. Journ. 
Geol. Soc.,’ vol. xxvii, p. 128. 
