88 SECTIONAL ADDRESSES. 
of the Downtonian rests upon a satisfactory basis. ‘Towards the close 
of the Silurian, as is perfectly well known, far-reaching changes in 
physical conditions took place, necessarily involving changes in the 
character of the shallow-water fauna whenever and wherever these 
occurred, and the coming in of fishes appears to be directly connected 
with them. That these changes took place simultaneously over wide 
areas is in the highest degree improbable, and having had some experi- 
ence of the behaviour of these rocks in the field I have felt that the 
evidence at times so strongly suggested that the Downtonian was essen- 
_ tially a facies formation that the possibility of its horizon being eventu- 
' ally found to be almost as inconstant as that of the Millstone Grit was 
far from improbable. That there may appear to be a similar change 
of conditions in parts of Britain and France at about the same time is 
not really the point; it is not the succession of shallow-water marine 
faunas that is important from the point of view of classification, but 
how far these are really of the same age in different places, and how 
much change is reflected in the fauna of the more stable deeper-water 
beds. The author may be perfectly right in his contention, only up 
to the present as I see the problem he has not proved his case. 
So, too, at the lower limit of the same Silurian formation; at 
' present the top of the Ashgillian needs clearer demarcation, and [ 
have endeavoured to show elsewhere that on paleontological grounds 
the most satisfactory place at which to draw the line is at the horizon 
where Monograptus makes its first appearance in force in the deeper- 
water sediments of the period, a well-defined faunal change indicative 
of the attainment of an important evolutional stage, of world-wide 
significance, and independent so far as can be determined of any change 
in the nature of the sedimentation. This appears also to be the horizon 
of the entrance in force of the true Pentamerids (Barrandella) amongst 
the faunas of shallow-water type. 
With regard to these general principles of modern classification, 
there would appear also to be only one really effective way of rescuing 
our Science from the increasing burden of local nomenclature; this has 
had its uses undeniably in indicating the exact nature of local successions 
and developments, and at present cannot be avoided in unfossiliferous 
rocks such as those of Pre-Cambrian age, but for the rest of our Lower 
Paleozoic rocks surely the time is coming, if, indeed, it has not already 
come, when there may be detected emerging from all this wealth of 
local detail a general paleontological sequence that may be of wide and 
possibly even of international application. 
There will, no doubt, be those who will object on the grounds that 
in adopting such a classification the geological world might be at the 
mercy of the whims of a few paleontologists; this should not be the 
case if the evolutionary principle be adopted, for the choice of fossil 
indices should then be limited to those fossils that are of the nature 
of stable or successful types, for these are likely to be the only forms 
with a sufficiently wide distribution in space to be really useful, whilst 
in widely remote areas, should these fail, corresponding forms at a 
similar general stage of evolution should be utilised in their stead. 
The most reliable classification for shallow-water beds would then 
