262 
PROCEEDINGS OF THE PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
Instead of altitude we might perhaps more appropriately | 
say temperature, since it is by reason of the latter that 
altitude has any influence. Au increase of altitude is 
similar, as regards the mean temperature, to an increase 
of latitude; as in both cases the mean temperature is 
diminished. 
Altitude (or temperatnre) influences distribution in two 
ways. Some species cannot keep a footing where the 
temperature is lower than a certain mean. Hence they 
cannot live above a certain altitude. Others, while capable 
of living in the same temperature as those mentioned, are 
also capable of living in a lower, and hence can also find 
a home at a higher altitude. Yet another set can live at 
the lower altitude, but being for some reason less fitted to 
maintain their own in the struggle for existence, are driven 
to a higher altitude, where the struggle is less intense, and 
are hence in a great measure confined to the higher alti¬ 
tude. On the other hand, some species seem unable to 
survive when the temperature rises above a certain mean, 
and are hence necessarily confined to the higher altitudes. 
The influence of altitude upon distribution is not confined 
to plants only, but may be studied in many groups of 
animals, and we in Perthshire are very favourably situated 
for investigating the subject. 
The connection of temperature with altitude leads us 
naturally to the question of the connection between the 
meteorology and the distribution. 
Temperature as influenced by altitude in a wide sense 
we have already considered. We must not, however, lose 
sight of temperature depending upon local conditions. The 
influences also not only of the amount of the rainfall, but 
of the amount of moisture Id the atmosphere;—of the 
extent of the snowfall, and the time during which it lies;— 
and of the prevailing winds, must all be taken into account, 
as there is but little doubt that each of these bears more or 
less directly upon the distribution of species. 
The river system of the county as an agency in directing 
or controlling the distribution must not be overlooked. In 
addition to the action of running water as a carrier of 
species—not only of plants but of animals—from one place 
to another, the banks of the rivers frequently afford such 
conditions as are favourable for an increase in the produc¬ 
tion of individuals, and a consequent spread of the species 
up and down the stream. On the other hand, wide rivers 
present barriers to the distribution, though none of our 
streams are probably sufficiently large to be of much im¬ 
portance in this respect, unless it be the Biver Tay in its 
lower course, where it is several miles in width. As 
barriers to distribution, our mountain ranges are certainly 
more effective than the rivers. 
The distribution of the flora is an important factor in the 
distribution not only of the fauna but of part of the flora 
itself. (Having discussed at some length, in my address 
as President in 1884 of the East of Scotland Union of 
Naturalists’ Societies, some of the causes of the distribu¬ 
tion of our plants, it seems unnecessary to enter into that 
subject just now more than has been already done in the 
preceding remarks.) 
In alluding to the agency of one part of the flora upon 
the distribution of the other part, I refer of course to those 
species which are either parasitic upon other species, 
living or dead, or, like maDy woodland plants, are in a 
great measure dependent upon the protection afforded by 
other vegetation. 
The effects of the distribution of the flora upon the dis¬ 
tribution of the fauna must be very great. The range of 
all the animals which derive their food from plants—and 
more especially of those species which are dependent upon 
particular plants—is regulated by the distribution of the 
flora. But the effect doe3 not terminate here, for since 
certain animals feed upon certain others, the causes that 
control the distribution of the latter will also control the 
distribution of the former. While the action of this agency 
is most potent and widely spread in the insects, yet 
all other departments of the fauna are more or less affected 
by it. 
On the other hand, the distribution of the fauna acts in 
several ways on the distribution of the flora, as, for 
example, by the agency of insects in the fertilization of 
plants; by the agency of birds and mammals in carrying 
seeds; and also by the destruction of plants or of essential 
parts of them by animals. It must not be forgotten either 
that birds probably occasionally act as carriers of living 
mollusks. 
Finally, there is to be considered the action of human 
agency on the distribution of species. Under this heading 
will come all the various effects of agriculture, of the 
reclamation of waste lands, of the formation of plantations 
and of railways, &c., &c., which, while preventing the in¬ 
crease or even causing the extinction of some species, are 
favourable to the introduction or increase of others. 
In thus briefly discussing some of the causes which regu¬ 
late the range of species, in order to show the necessity of 
a thorough investigation of the local distribution, I have 
abstained from giving illustrations under each heading. It 
would have been comparatively easy to have done so—and 
in fact I had the notes—had the limits of this address per¬ 
mitted. But since I have still to ask your attention to 
another and even more important reason why we should 
have in the Museum a large series of specimens, I have 
