PROCEEDINGS OF THE PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
263 
thought it desirable to omit all illustrative remarks. 
Before quitting the subject of the laws of distribution I 
should perhaps point out—though I daresay it is sufficiently 
evident—that while one or other of the various agencies to 
which allusion has been made, may be predominant in its 
eSects on the distribution of a species, yet in most cases 
there will be more than one agent. 
I now pass on to the other, and—on this occasion—final 
argument for the necessity of having in the Museum 
numerous examples of each species, collected in various 
parts of the county. 
There is in every species a greater or less degree ef in¬ 
stability; that is to say, no two individuals of a species are 
exactly alike in every respect, while in some species the ex¬ 
tent to which individuals differ from one another is very 
great. To enable us, therefore, to properly understand a 
species, it is necessary that it be illustrated by a more or 
less numerous series of examples. 
The variability of species is a subject that deserves much 
attention. In investigating this subject there are two 
questions to be considered :— 
1st, The degree and nature of the variation. 
2nd, The causes of it. 
These again are capable of subdivision. Under the former 
we have firstly to ascertain to what extent our local 
specimens vary amongst themselves; and, secondly, to find 
out if, and how, they differ from specimens obtained in other 
districts or countries. Of these we can and ought to 
investigate the former for ourselves, but as in our Museum 
we have not the necessary material for comparison, the 
latter can scarcely be altogether satisfactorily studied by 
ourselves. At the same time this ought not to deter us 
from obtaining the specimens, for, as I have already 
endeavoured to explain, we form the Museum not for our 
own advantage merely, but for the benefit of naturalists 
in general. We must provide the material even though we 
cannot ourselves utilise it to the fullest extent. 
The second division of this subject,—namely, the causes 
of the variation,—is a much more complex one. For its 
study, the circumstances and conditions under which the 
varieties occur must be studied and compared with the 
like facts of the occurrence of typical individuals of other 
varieties of the same species. But though it is complex it 
is not to be doubted that, by the accumulation of facts 
and the careful study thereof, the problem will be some 
day solved. 
Let me indicate briefly some of the lines in which 
variation runs in Perthshire species. Time will not of 
course permit of even a glance at all the groups of animals 
and plants, so I will confine myself to one or two. 
Of the insects, the butterflies and moths present us with 
some marked forms of variation^. It is usually thought 
—and in most cases with reason—that when northern and 
alpine examples of these insects differ from southern and 
lowland individuals of the same species, the variation is 
in the direction of a tendency to infuscation, or suffusion 
with black, often to such an extent that the markings are 
more or less obliterated. Such varieties are termed from 
their colour “melanic.’’ This form of variation is so 
generally the case that it may almost be said to be the rule 
for northern and alpine insects to present melanic forms, 
and if there were no exceptions to it, it might be less 
difficult to discover the cause. As it is, various theories 
have been propounded, but none of them seem to me 
altogether satisfactory. The exponents of these theories 
have overlooked in great measure, or altogether, the fact 
that some of the northern variations are the very opposite 
of melanism,—the insects being invariably much paler in 
colour than their southern relatives. 
In studying this subject in Perthshire,—and looking at 
the physiography of the county it will be admitted that we 
have a sufficiently wide field for profitable investigation,— 
we must first discover whether Lowland and Highland 
specimens differ, and hoiv they differ ; and, in the second 
place, when a difference exists, whether it is sufficiently 
constant and universal as to entitle the specimens that 
show it to be considered as forming a local race. After 
this has been found out—and it is only by getting together 
numerous specimens from various parts of the county that 
we can expect to do so—it will be time to try to discover 
the causes that lead to the variation. 
Another line in which variation runs is that of size. 
Some of our species are invariably smaller, others are 
larger, than southern individuals, and possibly we may find 
a similar difference between Highland and Lowland speci¬ 
mens. The shells of many of our mollusca are, I am in¬ 
clined to think, rather larger than English examples of the 
same species. Is this the case ? and, if it is, what is the 
cause ? Again, it is often thought that Highland specimens 
of some of our common plants have larger flowers, as well 
as brighter colours, than Lowland examples. Is this so, 
and what is the reason? As I have already said, a large 
series of local examples must be collected, and when we 
have got our data we can then begin to theorise. 
But it is time to conclude this address, in which I regret 
I have been able merely to touch several subjects of high 
interest, since my primary object has been to endeavour to 
shew that our present duty is the aggregation of facts, and 
that an important method of so doing is the aggregation of 
properly-selected specimens in the Museum. We must 
