ON THE WORLD-DISTRIBUTION OF BRITISH PLANTS. By Thomas Comber, Esq. (BEAD JANUABY 23, 1873.) THE geographical distribution of British plants has long had the careful attention of our botanists, and been the subject of their patient study. The results attained have been recorded, in a manner which leaves nothing to be desired, by Mr. H. C. Watson in his Cybele Briltanica, and in a more condensed form in the Compendium of that work. The recorded facts accumulated by Mr. Watson furnish, as he himself remarks, a fresh starting point for other workers in the same field; and provide a store of materials which can bo used with every confidence as to their accuracy. As regards their occurrence within Britain, Mr. Watson has proposed for British plants certain groups, which he terms types of distribution ; such as British for those plants which are met with pretty generally all over Great Britain ; Scottish and English for those which are found only or mostly in the Northern or Southern half of the island; German and Atlantic for those which are confined chiefly to the South-eastern or South-western provinces : but, although from the names of the two last it might be inferred that the range outside of the United Kingdom is indicated, Mr. Watson is careful to state that his types are " to be understood in " reference only to their distribution within Britain itself and " by itself." So far as I am aware no attempt has yet been 238 made to arrange our plants into groups according to the general geographical area they occupy outside of the United Kingdom : and it is to suggest such a grouping that I have to solicit your attention to-night. Important as the subject of Botanical Geography is to naturalists, who hope from a patient investigation of its facts to arrive eventually at a knowledge of the laws which govern the spread of species, and perhaps even to find some key which will help them to solve the mystery of the origin of species, and enable them to decide between the rival theories of special creation or natural development still I fear that to many of my hearers the subject of my paper will not be of any great interest, and I must trust to their forbearance if they find me somewhat dry. The plan I propose to follow is first to describe the groups into which it seems to me that the members of our Flora may most naturally be arranged, defining the foreign area over which each group ranges, giving the number of species which it contains, and as examples a few instances taken as far as possible from our commonest and best-known species. Following this will come detailed lists of species, in which will be distinguished, by being printed in italics, all such plants as are known or suspected to be only naturalised in Britain, or whose foreign area is uncertain, owing to their being extensively naturalised in other countries. At first I thought of omitting altogether plants whose area of distri­ bution has been thus interfered with by human agency; but this would exclude, amongst our naturalised plants, many whose presence with us is of great interest, either from their introduction having been quite recent, their invasion having taken place as it were under our own eyes, as in the case of the American water weed (Elodea Canadensis) and Bux- baum's Speedwell (Veronica Buxbaumii} ; or from the plants having been settled with us for so long a period that they 239 have come to be regarded almost as true natives, as the Poppies and Fumitories. The lists, therefore, include all aggregate species admitted into the sixth edition of the London Catalogue, except a few, the area of which, from confused nomenclature or other causes, has not been satis­ factorily determined. This will probably be as much as can be done to-night; in a future paper I hope to compare the results arrived at with certain propositions laid down by Prof. Alph. De Candolle in his " Geographic Botanique;" and to remark on the different degrees of distribution shown by plants of certain classes and orders, or having peculiar habitats or modes of seeding. According to latitude and elevation four ZONES may be recognised, viz. : I. Southern: Comprising plants which belong chiefly to the latitude of the Mediterranean ; and, although they extend as far as Britain, are not found North of the parallel of 60° ; nor with us at a higher elevation than Mr. Watson's Agrarian region. II. Temperate : Containing plants which, while generally extending South to the Mediter­ ranean, range over the whole of Europe, except frequently the extreme North ; and also some plants of more limited range, which extend neither far North nor far South. III. Northern : Consisting of species of Northern and Central Europe, which, although descending with us into the Agrarian region, if they reach Southern Europe are then found only at a considerable elevation. IV. Arctic: Including plants of the extreme North, which do not descend to the sea level in any part of Britain ; and also those which extend further South, but are there strictly Alpine plants. Viewed as to their range from East to West, our Flora falls likewise into four main divisions, extending as follows: Division I. To Europe only : not spreading further East than the Ural Mountains and the Caucasus, nor extending to any part of Asia, except the country immediately bordering the Mediterranean. Division II. To Europe and Asia : ranging more or less into the latter continent, and sometimes stretching quite across to its Eastern shores. None of the species in these two first divisions are met with in America, except as introduced plants or emigrants; but some of them extend Westwards to the islands of the Atlantic ; the Southern species to the Azores, Madeira, and the Canaries ; the Nor­ thern chiefly to the Faroe Islands and Iceland ; and they thus form an intermediate step towards the next two divisions, the plants contained in which are found in the New World as well. Some of these are also to a certain degree intermediate, being found in America only in Greenland, whose Flora is semi-European in type. Others seem to have spread into America from Asia rather than from Europe, being recorded only West of the Eocky Mountains ; and a few occur only in the Aleutian Islands, which occupy the same intermediate position on the Paci-fic that Iceland and Greenland do on the Atlantic side. Division III contains the comparatively small number of species that are confined to Europe and America, but have not yet been found in Asia: while Division IV, which, for want of a better term, may be called Universal, comprises plants which spread into all the three continents of the Northern Hemisphere. Most of them do not extend into or across the tropics; but there are a number which more properly deserve to be called " Universal," for they are met with in the Southern Hemisphere; and some are true cosmo­ politans, being found almost all over the globe, and apparently equally at home everywhere. Combining these four divisions with the four zones pre­ viously described, we have sixteen primary groups, some of which contain only a small, others a very large, number of species. These latter may conveniently be divided into sections, according as plants composing them occupy partially or wholly the area of the groups. 241 GROUP I. Soutltern Europe. This group comprises 223 species, which mostly belong to the Mediterranean district, and are generally more abundant there than with us. Many of them (87, or 39 per cent.) are recorded from the North of Africa; and a smaller proportion (53, or 24 per cent.) have been found in the Azores, Madeira, or the Canaries ; 77 (or 36 per cent.) are colonists, or otherwise naturalised. I propose to arrange them into four sections. Section 1. Twenty species which are found in the Spanish Peninsula, and thence range Northwards to the British Islands, Statice bahtmensis, and perhaps the Sea Radish (Kaj)hanuft maritimm), being also reported from the Southern extremity of Scandinavia. The London Pride, so commonly cultivated in gardens (Saxifraga umbrosa), has ii peculiar distribution, as it occurs in the Pyrenees and in Ireland, but has not been detected in any intermediate station. This, and many others in subsequent sections, are classed as Southern plants with great hesitation, for although their area of distribution is altogether to the South, they exhibit a tendency to become to a certain extent mountain plants on the Continent. Other well-known plants of this section are two of our Heaths (Erica ciliaris and E. vagans), which are never found far inland from the Atlantic. Section 2. Eighteen species which do not extend East­ ward along the Mediterranean beyond Italy, and in crossing Europe are not found East of Germany proper. The Isle of Man Cabbage (Sinapis Cheiratithus) serves as an example. Section 3. Sixty-four species which either range along the Mediterranean beyond Italy, or in passing North stretch into Austria, but are not found in Eussia, such as the Cheddar Pink (Uianthus ctesius), and the Wild Hyacinth (Hyacin- thus nonscriptus). Section 4. One hundred and twenty-one species which pass the frontiers of Eussia Eastward. Many of them extend even to 24% the Province of the Caucasus, and thus, closely approaching Asia, form a transition to the first Section of Group V. The Musk Mallow (Malva moschata), and the Lords and Ladies (Arum maculattim), are examples of those which do not; the Trailing St. John's Wort (Hypericum humifusum), many of our Clovers, and the Foetid Iris (/.