PROCEEDINGS OF THE PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
261 
These, then, are the two great divisions, and that they 
are well marked can be readily seen any clear day from 
Kinnoull Hill or other convenient standpoint. The base of 
the Grampian Hills, stretching from north-east to south¬ 
west, plainly indicates the boundary between the two 
areas. 
The districts —thirteen in number—are obtained by sub¬ 
dividing the two primary divisions according to the 
drainage system. It is not necessary to particularise them 
just now. 
Prom this brief sketch it will be admitted, I think, that 
the physical features of Perthshire ought to affect the 
distribution of the fauna and flora. Of course the mere 
contour of the surface can play in itself but a secondary 
part, but as influencing the meteorological conditions, and 
as presenting the appropriate habitats, it is of great im¬ 
portance. 
Before attempting to ascertain what the laws of dis¬ 
tribution are, it will be necessary to find out what is the 
distribution of each species, and I now repeat, what I 
have already said, that in order to avoid any errors it is 
extremely desirable that properly-authenticated specimens 
should be obtained and preserved from every district. 
When that has been done, and not till theD, we will be in 
a position to conduct satisfactory enquiries into the reasons 
for the ascertained distribution. In the meantime I may 
allude to some, of the factors that probably more or less 
affect the distribution of species, and in thus alluding to 
these I am chiefly actuated by a desire to induce some of 
you to look into the matter and investigate it for your¬ 
selves, remembering that every fact ascertained and re¬ 
corded will always be a help to the elucidation of the 
problem. 
First, as to the Petrology, that is, the nature and mineral 
constituents of the rocks. 
Some naturalists are inclined to doubt that on the whole 
there is much connection between the petrology of a 
district and its fauna and flora, and perhaps they are to a 
certain extent right. But that there is in some cases a 
distinct relation cannot be denied, and the extent of this in 
Perthshire is a subject that deserves investigation. 
The connection between the petrology and the distribu¬ 
tion of species must be considered in several aspects. For 
example, we must enquire as to whether the mineral con¬ 
stituents of the rocks have any bearing on the presence or 
absence of certain species. At present I am aware of one 
constituent only that has a distinct influence, and that is 
lime. A very few of our plants occur only upon lime¬ 
stone, and where that is absent it is of no use looking for 
them, though they are not always present where limestone 
occurs. While thus associated with limestone, a super¬ 
abundance of lime does not seem to be essential for their 
growth, as they may be readily cultivated in ordinary 
garden soil. Of animals I am not aware that we have 
any that are thus restricted, though certain of our 
mollusca seem to be more abundant where limestone is 
present. Limestone, though it cannot be termed 
abundant in Perthshire, yet occurs here and there over a 
large area, and it is desirable that all the localities where 
it does occur should be carefully examined. Whether it is 
the presence of lime in the porphyritic rocks of the Sidlaw 
and Ocbil ranges, and in the conglomerates of the High¬ 
land border, that makes them so much more productive 
than the sandstone rocks, is a matter for enquiry. It is at 
anyrate certain that some of our plants are almost or 
quite restricted to these, and they appear to be also richer 
in mollusca. 
The form and degree of hardness of the rocks has a not 
unimportant bearing upon the distribution of species of 
plants at least. Rocks of a porphyritic nature like those 
of the Sidlaw hills, which form steep slopes and precipices, 
afford greater opportunities for species to safely establish 
themselves, than those which form or have been shaped 
into rounded masses, with few irregularities of surface. 
And when added to the precipitous character we find a 
greater or less tendency to disintegration, the most 
favourable conditions for plant life—and as we shall notice 
presently for other forms of life also—are afforded apart 
from the chemical and other influences. Hence probably 
the greater fertility of some parts of the porphyritic hills 
and of some of the Highland mountains. 
The temperature and the drainage of the rocks are also 
matters that must be taken into account. In considering 
the former, and putting aside all question of the aspect,— 
since a locality that faces the sun must necessarily be more 
favoured than one that doesnot,—I am inclined to think that 
a porphyritic rock—be it from its colour or from its texture 
—absorbs more sun-heat than a sandstone rock, and that 
this is one reason why some sheltered nooks of the Sidlaws 
are so comparatively rich. 
The effect of the form of the rocks upon the drainage, 
and hence upon plant life, is better seen in the Highlands 
than in the Lowlands, though examples can be found 
everywhere. A hill that is furnished with moderately 
damp rocks will—other things being equal—always be 
found more fertile than a hill with dry rocks. Hence it is 
that some of our Highland districts are so much more 
productive than others. 
Altitude above sea-level is an important factor in the dis¬ 
tribution of species, not only of plants but of animals. 
