PLATE XXIX. 
No. 1.—The Dark Argus Butterfly (Polyommatus Sal- No. 4.—The Scotch Argus Butterfly (Polyommatus 
macis) . Artaxerxes). 
No. 2.—The Female of the Dark Argus. No. 5.—The Female of the Scotch Argus. 
No. 3.—The Dark Argus, showing the Under side. No. 6.—The Scotch Argus, showing the Under side. 
By many British Entomologists the two species figured in this Plate are considered mere 
varieties of Polyommatus Agestis. It is, however, very difficult to draw a line which shall cor- 
rectly and satisfactorily separate certain slight deviations from a generic type, into distinct 
“species” on the one hand, and mere “ varieties” on the other. In some cases the distinctions 
produced merely by climate and other local causes are allowed to constitute perfect and specific 
characteristics, while in others, equally striking differences are treated as mere variations. It 
would seem that where particular characteristics are constantly and regularly transmitted from 
the parent to the offspring, the peculiarities, not being variable, must be considered as marks of 
distinct species. On the other hand, when any such peculiarities of colour or structure are evi- 
dently accidental, and occurring in broods where the majority are of the usual kind, then, how- 
ever striking the differences may appear, they can only constitute “ varieties.” To meet the 
first of these cases, the term “ permanent variety” has been invented, while the insects clearly 
belonging to the second category are termed simply “varieties.” But it may be fairly assumed 
that a “ permanent variety” is precisely equivalent to a species. Such “ permanent varieties” 
might be tested as to their claim to be considered species by removing broods of them to the 
localities where the typical species abound, and if their distinctive characteristics remain unim- 
paired, which they are very likely to do, although originally produced by local causes—then 
there could be no longer any doubt as to their claim to be considered distinct species, for it is 
no doubt to local influences, acting through a long series of ages, that many of the most striking 
“specific” characteristics have been produced. 
If such a line of argument be admissible, then, till further investigation throws more light 
on the subject, Polyommatus Salmacis, which is only found in our northern counties, must be 
considered as a distinct species, inasmuch as its chief characteristics are permanent, and would 
probably remain so, at all events for a long series of generations, if a colony of it could be es- 
tablished in a more southern situation. The same may be said of P. Artaverxes. It is, how- 
ever, but fair to add, that gradations of these disputed species occur in intermediate localities, 
seeming to afford links between these two kinds, which would prove a very close relationship. 
Mr. Newman in the “ Entomological Magazine,” states, for instance, that, as the Brown Argus 
of the metropolitan districts advances to the midland counties, an evident change takes place, 
and the band of rust-coloured spots becomes less bright. At Manchester these spots have nearly 
left the upper wing; at Castle Eden Dene they are scarcely to be traced, and a black spot in the 
centre of the upper wing becomes fringed with white, being in some specimens quite white ; 
the Butterfly then changes its name to Salmacis. As we proceed further northward, the black 
pupil leaves the “eyes” on the under side, until, at Edinburgh, it is quite gone, and then the 
insect is called Artaxerzes.* 
#* My. Gardner informs us that all three species have been taken at Castle Eden Dene. 
