Vice-President’s Address. 191 
Again, we find very few of the Lower Coal-Measure plants 
passing up into the Upper Coal-Measures, and even the 
few that do so pass up are among the rarest of Upper Coal- 
Measure species, whereas they are the commonest species 
in the Lower Coal-Measures: even from the Middle Coal- 
Measures very few pass into the Upper Coal-Measures, The 
Middle and Lower Coal-Measures have a much greater 
similarity in their flora, but here, too, we find each series 
characterised by its fossil plants. 
But before passing from the more general questions which 
affect the whole subject of the value of paleontological 
evidence in determining the age of rocks, the very important 
question as to the practical application of the principle 
demands some attention, for on the use we make of our tools 
depends the value of the work done. 
Some paleontologists have accepted the occurrence of a 
single species or even specimen as sufficient evidence for the 
determination of a horizon, This is an abuse of paleontology 
—and such treatment of fossils has brought the science into 
disrepute, if not contempt, in some quarters; but for this 
paleontologists, and not palzontology, are to blame. 
From the occurrence of Alethopteris (Pecopteris) Serlii, 
Brongt. sp., in the Forest of Wyre, Worcestershire, the late 
Dr Stur correlated these beds with the Upper Coal-Measures.1 
Now, though Alethopteris Serlia is a characteristie Upper Coal- 
Measure plant, and is extremely abundant in that horizon, 
it also occurs, though very rarely, in the Middle Coal- 
Measures. If instead of fixing upon a single species as the 
test of age, the other fossils found associated with Alethopteris 
Serlii in the Forest of Wyre had been taken into account, it 
would have been evident that the Coal-Measures of the Forest 
of Wyre were of Middle Coal-Measure age. 
In fossil botany, it is utterly unsafe to fix upon one species 
for determining horizons, and altogether to ignore the other 
plants with which it is associated. It must always be borne 
in mind that the presence of certain species is not always 
of itself sufficient data for the determination or correlation of 
horizons, for equally important is the absence of others. 
1 Verhandl, d. k. k. geol. Reichs., 1884, No. 7, pp. 135-141. 
