Island Life Part 20

65. ,, DISTINGUENDA (Rye). South of England; many specimens.

66. PSYLLIODES LURIDIPENNIS (Kutschera). Lundy Island. A very curious form, not uncommon in this small island, to which it appears to be confined. "An extreme and local variety of _P. chrysocephala_"

(Fowler).

COCCINELLIDae.

67. SCYMNUS LIVIDUS (Bold). Northumberland. A doubtful species.

Of the sixty-seven species and varieties of beetles in the preceding list, a considerable number no doubt owe their presence there to the fact that they have not yet been discovered or recognised on the continent. This is almost certainly the case with many of those which have been separated from other species by very minute and obscure characters, and especially with the excessively minute Trichopterygidae described by Mr. Matthews. There are others, however, to which this mode of getting rid of them will not apply, as they are so marked as to be at once recognised by any competent entomologist, and often so plentiful that they can be easily obtained when searched for. The peculiar species of Apion in the Shetland Islands is interesting, and may be connected with the very peculiar climatal conditions there prevailing, which have led in some cases to a change of habits, so that a species of weevil (_Otiorhynchus maurus_) always found on mountain sides in Scotland here occurs on the sea-sh.o.r.e. Still more curious is the occurrence of two distinct forms (a species and a well-marked variety) on the small granitic Lundy Island in the Bristol Channel. This island is about three miles long and twelve from the coast of Devonshire, consisting mainly of granite with a little of the Devonian formation, and the presence here of peculiar insects can only be due to isolation with special conditions, and immunity from enemies or competing forms. When we consider the similar islands off {355} the coast of Scotland and Ireland, with the Isle of Man and the Scilly Islands, none of which have been yet thoroughly explored for beetles, it is probable that many similar examples of peculiar isolated forms remain to be discovered.

Looking, then, at what seem to me the probabilities of the case from the standpoint of evolution and natural selection, and giving due weight to the facts of local distribution as they are actually presented to us, I am forced to differ from the opinion held by our best entomological authorities, and to believe that some at least, perhaps many, of the species which, in the present state of our knowledge, appear to be peculiar to our islands, are, not only apparently, but really, so peculiar.

I am indebted to Mr. Robert McLachlan for the following information on certain Trichopterous Neuroptera (or caddis-flies) which appear to be confined to our islands. The peculiar aquatic habits of the larvae of these insects, some living in ponds or rivers, others in lakes, and others again only in clear mountain streams, render it not improbable that some of them should have become isolated and preserved in our islands, or that they should be modified owing to such isolation.

_Trichoptera peculiar to the British Isles._

1. PHILOPOTAMUS INSULARIS. (? A variety of _P. monta.n.u.s_.)--This can hardly be termed a British species or variety, because, so far as at present known, it is peculiar to the Island of Guernsey. It agrees structurally with _P. monta.n.u.s_, a species found both in Britain and on the continent, but it differs in its strikingly yellow colour, and less p.r.o.nounced markings. All the specimens from Guernsey are alike, and resident entomologists a.s.sured Mr. McLachlan that no other kind is known. Strange to say, some examples from Jersey differ considerably, resembling the common European and British form. Even should this peculiar variety be at some future time found on the continent it would still be a remarkable fact that the form of insect inhabiting two small islands only twenty miles apart should constantly differ; but as Jersey is between Guernsey and the coast, it seems just possible that the more insular conditions, and perhaps some peculiarity of the soil and water in the former island, have really led to the production or preservation of a well-marked variety of insect. In the first edition of this work two other species were named as then, peculiar to Britain--Setodes argentipunctella and Rhyacophila munda, but both have now been taken on the continent.

2. MESOPHYLAX IMPUNCTATUS, _var._ ZETLANDICUS.--A variety of a South and Central European species, one specimen of which has been found in Dumfriesshire. The variety is distinguished by its small size and dark colour.

{356}

_Land and Freshwater Sh.e.l.ls._--In the first edition of this work four species were noted as being, so far as was then known, exclusively British.

Two of these, _Cyclas pisidioides_ (now called _Sphaerium pisidioides_) and _Geomalacus maculosus_, have been discovered on the continent, but the other two remain still apparently confined to these islands; and to these another has been added by the discovery of a new species of Hydrobia in the estuary of the Thames. The peculiar species now stands as follows:--

1. LIMNEA INVOLUTA.--A pond snail with a small polished amber-coloured sh.e.l.l found only in a small alpine lake and its inflowing stream on Cromagloun mountain near the lakes of Killarney. It was discovered in 1838, and has frequently been obtained since in the same locality. It is sometimes cla.s.sed as a variety of _Limnea peregra_, and is at all events closely allied to that species.

2. HYDROBIA JENKINSII.--A small sh.e.l.l of the family Rissoidae inhabiting the Thames estuary both in Ess.e.x and Kent. It was discovered only a few years ago, and was first described in 1889.

3. a.s.sIMINEA GRAYANA.--A small estuarine pulmon.o.branch found on the banks of the Thames between Greenwich and Gravesend, on mud at the roots of aquatic plants. It has been discovered more than sixty years.

But besides the above-named species there are a considerable number of well-marked varieties of sh.e.l.ls which seem to be peculiar to our islands. A list of these has been kindly furnished me by Mr. Theo. D. A. c.o.c.kerell, who has paid much attention to the subject; and after omitting all those whose peculiarities are very slight or whose absence from the continent is doubtful, there remain a series of forms some of which are in all probability really endemic with us. This is the more probable from the fact that an introduced colony of _Helix nemoralis_ at Lexington, Virginia, presents numerous varieties among which are several which do not occur in Europe.[84] The following list is therefore given in the hope that it may be useful in calling attention to those varieties which are not yet positively known to occur elsewhere than in our islands, and {357} thus lead, ultimately, to a more accurate knowledge of the facts. It is only by obtaining a full knowledge of varieties, their distribution and their comparative stability, that we can ever hope to detect the exact process by which nature works in the formation of species.

LIST OF THE SPECIES AND VARIETIES OF LAND AND FRESHWATER Sh.e.l.lS WHICH, SO FAR AS AT PRESENT KNOWN, ARE BELIEVED TO BE PECULIAR TO THE BRITISH ISLES OR NOT FOUND ON THE CONTINENT.

LIMACIDae.

1. Limax marginatus, _var._ MACULATUS. Ireland; frequent, very distinct.

2. ,, ,, ,, DECIPIENS. Ireland and England.

3. ,, flavus, _var._ SUFFUSUS. England; Melanic form.

4. ,, ,, ,, GRISEUS. England; Melanic form.

5. Agriolimax agrestis, _var._ NIGER. Yorkshire. Melanic. Azores.

6. ,, ,, ,, GRISEUS. England. Melanic.

7. Amalia gagates, _var._ RAVA. W. of England.

8. ,, sowerbyi, _var._ RUSTICA. England.

9. ,, ,, ,, NIGRESCENS. Surrey and Middles.e.x.

10. ,, ,, ,, BICOLOR. Ealing.

11. Hyalina crystallina, _var._ COMPLANATA. Near Bristol.

12. ,, fulva, _var._ ALDERI.

13. Vitrina pellucida, _var._ DEPRESSIUSCULA. S. England, Wales.

HELICIDae.

14. Arion ater, _var._ ALBO-LATERALIS. England, Wales, Isle of Man; very distinct.

15. ,, hortensis, _var._ FALLAX. England. Common at Boxhill.

16. GEOMALACUS MACULOSUS. Kerry and Cork. Three varieties have been described, one of which occurs in Portugal.

17. Helix aspersa, _var._ LUTESCENS. England. Not rare perhaps in France.

18. ,, nemoralis, _var._ HIBERNICA. Ireland.

19. ,, rufescens, _var._ MANCHESTERIENSIS. England.

20. ,, hispida, _var._ SUBGLOBOSA. England.

21. ,, ,, ,, DEPILATA. England.

22. ,, ,, ,, MINOR. England, Ireland.

23. ,, granulata, _var._ CORNEA. Lulworth, Dorset.

24. ,, virgata, _var._ SUBAPERTA. Bath.

25. ,, ,, ,, SUBGLOBOSA. England, Wales, Bantry Bay.

26. ,, ,, ,, CARINATA. Wareham, Dorset.

27. ,, caperata, _var._ MAJOR. England, Wales, Scotland. Distinct.

28. ,, ,, ,, NANA. England.

29. ,, ,, ,, SUBSCALARIS. Wales, Ireland.

30. ,, ,, ,, ALTERNATA. England, Kent.