THE PHYSICAL GEOGRAPHY OF THE PAST. 
199 
but they have a far greater importance than this in 
affording material to the philosophical naturalist for 
studying geographical distribution, and for working out all 
those great influences of climate, soil, and general external 
conditions, upon the varietal changes which occasionally 
become stereotyped as new species. In order to facilitate 
this highest aim of the naturalist our local societies ought, 
I think, to make their lists more quantitative in character, if 
that is possible, and to note more than they do at present 
the inter-dependence of animal and vegetable life, and the 
relations of the plants to the geological nature of the soil 
upon which they a,re found. The field geologist often gets 
valuable hints as to the character of the rocks hidden under 
a thick mantle of vegetation by observing the nature of the 
plants growing upon them. Plenty of instances of this kind 
must occur to any geologist who has occupied himself with 
the minute survey of a district. As a good example, I may 
mention that Professor A. H. Green, during his survey of the 
Carboniferous Rocks of North Derbyshire, found that he was 
often able to define the boundary of the Carboniferous Shales 
and Sandstones by the fact that rushes are found on the shales, 
and that heath and furze grow more plentifully on the sand¬ 
stones ; and he also notes that a crowded belt of the little 
Viola lutea is often seen along the outcrop of a sandstone bed, 
whilst not a single plant will be found on the shales that 
come out on either side. 
The aid which botany is able to afford to geology is 
manifestly reciprocal, and if botanists would only take care 
to note, amongst other things, the kind of sub-soil upon which 
any particular plant grows, they would, I think, render their 
lists more valuable and attach to them a far wider interest 
than they commonly possess. 
These are mere suggestions which I venture to make for 
your consideration, but I will not enlarge upon them this 
evening, as I wish to occupy your time in trying to deduce 
from a mass of geological facts, which to many of you must 
seem dry and uninteresting, certain generalisations which, 
I trust, will prove of interest even to those who have no 
special knowledge of geology. 
The student of geology, whilst occupied in observing the 
thickness of strata, their physical nature, order of super¬ 
position, angles of dip, and fossil contents, finds, it is true, a 
keen and vivid enjoyment in such work, but if he is of a 
philosophical turn of mind he will place but little scientific 
value upon such observations when they are regarded as mere 
isolated phenomena; but when a large mass of facts has 
