Ee er rrr 
C.—GEOLOGY. 89 
be that based upon the evolutional sequence of the members of one or 
more species-groups® belonging to gemera possessing considerable 
possibilities of variation (variation gradient) so long as such members 
continue to be important and characteristic members of the fauna; when 
they fail in this respect they should be replaced by the members of 
another species-group that succeeds in importance. Thus, as will be 
shown in the sequel, the evolutional series comprised within the species- 
group of Calymene cambrensis might well be adopted for classifying 
the lower part of the Ordovician in our own country, whilst in the upper 
part members of the species-group of C. blumenbachi might be utilised. 
It may prove advisable in some cases to choose genera belonging to two 
distinct phyla to serve as a check upon each other such as, in the case 
of certain of the Lower Paleozoic rocks, might be afforded by Trilobita 
and Brachiopoda; in other cases species-groups of either of these phyla 
might prove sufficient. 
Deeper-water Faunas of the Lower Palzeozoic. 
We must now pass on to the consideration of some Lower Paleozoic 
Faunas, and see what has been achieved by regarding them from the 
evolutional standpoint; and since, for reasons already given, it would 
seem that the faunas of the deeper waters must be taken as the standard 
for purposes of classification, these will be considered first. 
Throughout the greater part of Lower Paleozoic time the Graptolite 
Shales constitute the typical deposit of the deeper waters of our epi- 
continental seas, the factors controlling their accumulation being not 
depth as such, but rather the factors that are closely associated with 
depth, especially quietness of the water, and absence of coarse sediment. 
Strictly speaking, I suppose the graptolite fauna does not belong to the 
Black Shale, since it is in all probability pseudo-planktonic; but owing 
to similar conditions governing its distribution the two are almost 
invariably associated and may be taken in that sense to belong; in any 
case, its occurrence is independent of those factors which make for 
heterogeneity in the faunas of the shallower waters, so that the 
Graptolite Shales furnish the standard sequence for purposes of 
classification. As regards the study of this highly interesting group of 
organisms, it is as well to note at the outset their extraordinarily favour- 
able position from the evolutionary standpoint ; for though we may not 
know the complete story of the whole class of the Graptolithina, since 
at present its actual beginning is uncertain, we do appear, at any rate, 
to have a more or less complete history of the more important order, 
the Graptoloidea, comprised within the rocks of Lower Paleozoic age ; 
so that here, if anywhere, we ought to be able to study the various forms 
in their true relationship to each other. To a large extent this can be 
done, and the honour of its first conception belongs to Nicholson and 
5 Species-group or gens may be considered to be the aggregate of all the 
species which possess in common a large number of essential properties and 
are continuously related in space or time. Vaughan, Q.J.G.S., 1905, vol. 1xi, 
p. 183 
