_Cause of Great Speciality in Fishes._--The reason why fishes alone should exhibit such remarkable local modifications in lakes and islands is sufficiently obvious. It is due to the extreme rarity of their transmission from one lake to another. Just as we found to be the case in Oceanic Islands, where the means of transmission were ample hardly any modification of species occurred, while where these means were deficient and individuals once transported remained isolated during a long succession of ages, their forms and characters became so much changed as to bring about what we term distinct species or even distinct genera,--so these lake fishes have become modified because the means by which they are enabled to migrate so rarely occur. It is quite in accordance with this view that some of the smaller lakes contain no fishes, because none have ever been conveyed to them.
Others contain several; and some fishes which have peculiarities of const.i.tution or habits which render their transmission somewhat less difficult occur in several lakes over a wide area of country, though only one appears to be common to the British and Irish lakes. {344}
The manner in which fishes are enabled to migrate from lake to lake is unknown, but many suggestions have been made. It is a fact that whirlwinds and waterspouts sometimes carry living fish in considerable numbers and drop them on the land. Here is one mode which might certainly have acted now and then in the course of thousands of years, and the eggs of fishes may have been carried with even greater ease. Again we may well suppose that some of these fish have once inhabited the streams that enter or flow out of the lakes as well as the lakes themselves; and this opens a wide field for conjecture as to modes of migration, because we know that rivers have sometimes changed their courses to such an extent as to form a union with distinct river basins. This has been effected either by floods rising over low watersheds, by elevations of the land changing lines of drainage, or by ice blocking up valleys and compelling the streams to flow over watersheds to find an outlet. This is known to have occurred during the glacial epoch, and is especially manifest in the case of the Parallel Roads of Glenroy, and it probably affords the true solution of many of the cases in which existing species of fish inhabit distinct river basins whether in streams or lakes. If a fish thus wandered out of one river-basin into another, it might then retire up the streams to some of the lakes, where alone it might find conditions favourable to it. By a combination of the modes of migration here indicated it is not difficult to understand how so many species are now common to the lakes of Wales, c.u.mberland, and Scotland, while others less able to adapt themselves to different conditions have survived only in one or two lakes in a single district; or these last may have been originally identical with other forms, but have become modified by the particular conditions of the lake in which they have found themselves isolated.
_Peculiar British Insects._--We now come to the cla.s.s of insects, and here we have much more difficulty in determining what are the actual facts, because new species are still being yearly discovered and considerable portions of Europe are but imperfectly explored. It often happens that an insect is discovered in our islands, and for some {345} years Britain is its only recorded locality; but at length it is found on some part of the continent, and not unfrequently has been all the time known there, but disguised by another name, or by being cla.s.sed as a variety of some other species. This has occurred so often that our best entomologists have come to take it for granted that _all_ our supposed peculiar British species are really natives of the continent and will one day be found there; and owing to this feeling little trouble has been taken to bring together the names of such as from time to time remain known from this country only. The view of the probable ident.i.ty of our entire insect-fauna with that of the continent has been held by such well-known authorities as the late Mr.
E. C. Rye and Dr. D. Sharp for the beetles, and by Mr. H. T. Stainton for b.u.t.terflies and moths; but as we have already seen that among two orders of vertebrates--birds and fishes--there are undoubtedly peculiar British species, it seems to me that all the probabilities are in favour of there being a much larger number of peculiar species of insects. In every other island where some of the vertebrates are peculiar--as in the Azores, the Canaries, the Andaman Islands, and Ceylon--the insects show an equal if not a higher proportion of speciality, and there seems no reason whatever why the same law should not apply to us. Our climate is undoubtedly very distinct from that of any part of the continent, and in Scotland, Ireland, and Wales we possess extensive tracts of wild mountainous country where a moist uniform climate, an alpine or northern vegetation, and a considerable amount of isolation, offer all the conditions requisite for the preservation of some species which may have become extinct elsewhere, and for the slight modification of others since our last separation from the continent. I think, therefore, that it will be very interesting to take stock, as it were, of our recorded peculiarities in the insect world, for it is only by so doing that we can hope to arrive at any correct solution of the question on which there is at present so much difference of opinion.
For the list of Coleoptera with the accompanying notes I was originally indebted to the late Mr. E. C. Rye; and Dr. Sharp also gave me valuable information as to the recent {346} occurrence of some of the supposed peculiar species on the continent. The list has now been revised by the Rev. Canon Fowler, author of the best modern work on the British Coleoptera, who has kindly furnished some valuable notes.
For the Lepidoptera I first noted all the species and varieties marked as British only in Staudinger"s Catalogue of European Lepidoptera. This list was carefully corrected by Mr. Stainton, who weeded out all the species known by him to have been since discovered, and furnished me with valuable information on the distribution and habits of the species. This information often has a direct bearing on the probability of the insect being peculiar to Britain, and in some cases may be said to explain why it should be so.
For example, the larvae of some of our peculiar species of Tineina feed during the winter, which they are enabled to do owing to our mild and insular climate, but which the severer continental winters render impossible. A curious example of the effect this habit may have on distribution is afforded by one of our commonest British species, _Elachista rufocinerea_, the larva of which mines in the leaves of _Holcus mollis_ and other gra.s.ses from December to March. This species, though common everywhere with us, extending to Scotland and Ireland, is quite unknown in similar lat.i.tudes on the continent, but appears again in Italy, the South of France, and Dalmatia, where the mild winters enable it to live in its accustomed manner.
Such cases as this afford an excellent ill.u.s.tration of those changes of distribution, dependent probably on recent changes of climate, which may have led to the restriction of certain species to our islands. For should any change of climate lead to the extinction of the species in South Europe, where it is far less abundant than with us, we should have a common and wide-spread species entirely restricted to our islands. Other species feed in the larva state on our common gorse, a plant found only in limited portions of Western and Southern Europe; and the presence of this plant in a mild and insular climate such as ours may well be supposed to have led to the preservation of some of the numerous species which are or have been dependent on it. Since the first edition was {347} published many new British species have been discovered, while some of the supposed peculiar species have been found on the continent. Information as to these has been kindly furnished by Mr. W. Warren, Mr. C. G. Barrett, Lord Walsingham, and other students of British Lepidoptera, and the first-named gentleman has also looked over the proofs.
Mr. McLachlan has kindly furnished me with some valuable information on certain species of Trichoptera or Caddis flies which seem to be peculiar to our islands; and this completes the list of orders which have been studied with sufficient care to afford materials for such a comparison. We will now give the list of peculiar British Insects, beginning with the Lepidoptera and adding such notes as have been supplied by the gentlemen already referred to.
_List of the Species or Varieties of Lepidoptera which, so far as at present known, are confined to the British Islands. (The figures show the dates when the species was first described. Species added since the first edition are marked with an asterisk.)_
DIURNI.
1. POLYOMMATUS DISPAR. "The large copper." This fine insect, once common in the fens, but now extinct owing to extensive drainage, is generally admitted to be peculiar to our island, at all events as a variety or local form. Its continental ally differs constantly in being smaller and in having smaller spots; but the difference, though constant, is so slight that it is now cla.s.sed as a variety under the name of _rutilus_. Our insect may therefore be stated to be a well-marked local form of a continental species.
2. Lycaena astrarche, _var._ ARTAXERXES. This very distinct form is confined to Scotland and the north of England. The species of which it is considered a variety (more generally known to English entomologists as _P. agestis_) is found in the southern half of England, and almost everywhere on the continent.
BOMBYCES.
3. Lithosia complana, _var._ SERICEA. North of England (1861).
4. Hepialus humuli, _var._ HETHLANDICA. Shetland Islands (1865). A remarkable form, in which the male is usually yellow and buff instead of pure white, as in the common form, but exceedingly variable in tint and markings.
5. EPICHNOPTERYX RETICELLA. Sheerness, Gravesend, and other localities along the Thames (1847); Hayling Island, Suss.e.x.
6. E. pulla, _var._ RADIELLA. Near London, rare (1830?); the species in Central and Southern Europe. (Doubtfully peculiar in Mr. Stainton"s opinion.) {348}
NOCTUae.
7. Acronycta euphorbiae, _var._ MYRICae. Scotland only (1852). A melanic form of a continental species.
8. AGROTIS SUBROSEA. Cambridgeshire and Huntingdonshire fens, perhaps extinct (1835). The _var._ _subcaerulea_ is found in Finland and Livonia.
9. Agrotis candelarum _var._ ASHWORTHII. South and West (1855).
Distinct and not uncommon.
10. Luperina luteago, _var._ BARRETTI. Ireland (1864).
11. Aporophyla australis, _var._ PASCUEA. South of England (1830). A variety of a species otherwise confined to South Europe.
12. Hydraecia nict.i.tans, _var._ PALUDRIS.
GEOMETRae.
13. Boarmia gemmaria, _var._ PERFUMARIA. Near London and elsewhere. A large dark variety of a common species.
14. *B. repandata, _var._ SODORENSIUM. Outer Hebrides.
15. *Emmelesia albulata, _var._ HEBRIDIUM. Outer Hebrides.
16. *E. albulata, _var._ THULES. Shetland Islands.
17. *Melanippe montanata, _var._ SHETLANDICA. Shetland Islands.
18. *M. sociata, _var._ OBSCURATA. Outer Hebrides. A dark form.
19. Cidaria albulata, _var._ GRISEATA. East of England (1835). A variety of a species otherwise confined to Central and Southern Europe.
20. EUPITHECIA CONSTRICTATA.. Widely spread, but local (1835). Larva on thyme.
21. *E. satyrata, _var._ CURZONI. N. Scotland.
22. *E. nanata _var._ CURZONI. Shetland Islands.
PYRALIDINA.
23. Aglossa pinguinalis, _var._ STREATFIELDI. Mendip Hills (1830). A remarkable variety of the common "tabby."
24. *Scoparia cembrae, _var._ SCOTICA. Scotland (1872).
25. *Myelois ceratoniae, _var._ PRYERELLA. North London (1871).
26. *Howoeosoma nimbella, _var._ SAXICOLA. England, Scotland, Isle of Man (1871).
27. *Epischnia bankesiella. Isle of Portland (1888).
TORTRICINA.
28. APHELIA NIGROVITTANA. Scotland (1852). A local form of the generally distributed _A. lanceolana_.
29. GRAPHOLITA PARVULANA. Isle of Wight (1858). Rare. A distinct species.
30. CONCHYLIS ERIGERANA. South-east of England (1866).
31. *BRACHYTaeNIA WOODIANA. Herefordshire (1882).
32. *Eupoecilia angustana, _var._ THULEANA. Shetland Islands.
33. *TORTRIX DONELANA. Connemara, Ireland (1890).
TINEINA.
34. TINEA COCHYLIDELLA. Sanderstead, near Croydon (1854). Unique!