CHAPTER VII

PAST AND PRESENT

The dependence of animals upon plants for the food by means of which they continue to inhabit the earth, which was pointed out on a previous page (75), shows that the plant world is older than the animal world; but the immense age of both can be appreciated only by a study of stratigraphical geology. The tens of thousands of feet of sedimentary rocks, laid down in slow succession on the floors of ancient seas and lakes, and still reposing layer upon layer, and no less the great gaps in the series produced when, raised into the air, deposition ceased, and thousands of feet of rock were slowly worn away and washed down again into the sea by the action of frost and wind and water, point to periods incalculably remote as measured by the standards which we apply to human history. A few thousands of years measures the span which separates us from the Neolithic Period; but to the geologist a million years is but a convenient unit for expressing, so far as any expression by our time-standards is possible, the huge periods with which he has to deal. And even when we get back as far as the oldest fossils will take us, we are still a long way from having reached the epoch when life on the earth originated. As we work backward and study the fossils of older and older rocks, the multitudinous assembly of plants and animals which fill the world to-day are replaced by other and more primitive forms, many groups approaching each other and merging in common ancestral types. But still, the very oldest fossil-bearing strata contain the remains of organisms already far up the ladder of evolution. The Lamp Shells (Brachiopods), Pteropods, Trilobites, and Worms of the ancient Cambrian rocks have clearly a long ancestral history. Plants are not so abundantly preserved in the rocks as the skeletons and shells of animals, on account of their softer nature; but in the oldest known plants it is again clear that we are dealing with forms by no means primordial. It is the more interesting, then, to note that many very lowly forms of life have come down to us from times immensely remote, and are still present on the earth in abundance, swarming in every sea and in every pond, or nestling in damp crevices of the land; while higher types of immense antiquity still mingle with the crowd of recent Seed Plants, some of them forming noble forest trees. Of especial interest, taking into account the wide distinction which exists between the higher animals on the one hand and the higher plants on the other, is it to find that there are still in existence organisms which are so much on the border-line between these two great groups of living things that they can be referred to one or other only with hesitation, clearly indicating that animal and vegetable life sprang from a common source. Take the group known as Mycetozoa or Myxomycetes. These names alone show the divergent views which men of science have held regarding them, _Myxomycetes_ signifying “slime-_fungi_,” while _Mycetozoa_ means “fungus-_animals_.” These remarkable organisms, of which over 180 species are found in the British Isles, begin life as tiny wind-borne spores. Under suitable conditions of moisture and heat, the spore swells, its wall cracks, and the contents--a tiny globule of protoplasm--creep out, develop a little tail or _flagellum_, which by lashing about propels the pear-shaped _swarm-cell_ through the drop of water in which it began life. The organism feeds by catching bacteria and other minute particles of organic matter, which are conveyed into the interior of the little mass of protoplasm and digested. The swarm-cells increase in number by division, and ultimately unite in pairs to form a _plasmodium_, which may, by union with other plasmodia, eventually attain a quite large size. In this naked protoplasmic mass a very remarkable rhythmic movement is set up, the granular protoplasm of the interior streaming rapidly along certain channels for about 1-1/2 minutes, when the motion is reversed and it streams in the opposite direction. The whole mass now creeps about in moist places, usually in the form of a network of branching veins, feeding as it goes, usually on dead vegetable matter. When fully developed the plasmodium creeps out into some more open spot and transforms itself into masses of spores enclosed in spore-cases, which vary much in different genera as regards size, shape, and colour, and are often borne on delicate stalks. When ripe, the spore-cases, or _sporangia_, open, and the spores are liberated into the air to be dispersed by wind and eventually to begin growth on their own

account. This story partakes about equally of incidents characteristic of the life-history of the lower animals and of the lower plants. The fruiting stage and the wind dispersal of the spores recall the arrangements familiar in the Fungi, and are not matched in any section of the animal kingdom; while the creeping plasmodium, devouring food as it goes, is entirely suggestive of animal life, and is not paralleled anywhere in the vegetable kingdom. There is no reason to look on the Mycetozoa as a group of animals which have taken on certain plant-like characters, any more than as a group of plants which have evolved certain animal characteristics: we appear to see in them a very ancient group which has come down to us from a time when plants and animals, as we know them, had not yet become differentiated.

Among plants, as distinguished generally from animals by the production and abundant use of chlorophyll and of cellulose, we have still existing on the Earth a range of forms extending from almost the most primitive organism that we can imagine up to the splendid Seed Plant, specialized in a hundred ways. Every pool, every soil, swarms with bacteria, the lowliest form of life--organisms exceedingly minute, exceedingly simple, and capable of existing under highly diverse conditions both physical and chemical. Thence we can trace an irregular ascending scale through the Fungi, the Algæ, Mosses, Horsetails, Ferns, and Club-mosses, to the Conifers, and on to the highest of the Seed Plants, which exceed in their beauty of structure and complicated life anything that has gone before them. In fact, as Theophrastus says, your plant is a thing various and manifold. And this existing vegetation with its thousand forms is but the present manifestation of the vital activity which has populated the earth during tens of millions of years. The oldest rocks which have been preserved to us in such a condition as to yield remains of plants and animals in a recognizable form are those known as Cambrian, the deposition of which occurred at a period which geologists have variously calculated as from, say, 20 to 100 or more millions of years ago. Yet even at that immensely remote period, life, both vegetable and animal, was already abundant and diverse, as well as highly organized. As Darwin long ago pointed out, the geological record does not go back nearly far enough to allow us any insight into the evolution of the earlier forms of life. Below the Cambrian rocks, as represented in these islands and in Europe generally, with their well-developed fauna, are tens of thousands of feet of strata which once, no doubt, were sediments at the bottom of the sea, and later on hardened into slates and sandstones in which were embedded remains of more primitive organisms; but these rocks have been so altered during the immense period of their existence by heat and pressure and the other vicissitudes to which the restless crust of the earth is subject that they now present a mass of granite-like material in which all trace of organic life has been destroyed. In America the rocks of corresponding age are better preserved, and have yielded a limited fauna displaying an already advanced stage of evolution. To account for the strange paucity of animal remains it has been suggested that the creatures of these earliest times were soft-bodied, so that after death they left no trace behind. It may be noted that the pre-Cambrian rocks contain beds of limestone and of carbon (in the form of graphite); such beds, in later rocks, are composed of organic materials, the limestones being formed of the skeletons of minute marine creatures, particularly Foraminifera, and the carbon deposits of the remains of plants.

In Cambrian times, then, abundant life springs forth into our vision from the rocks, already, like Minerva, fully armed. The soft plant structures are not well preserved in the older fossiliferous rocks, and hence the fragmentary story of plant life, as we trace it backwards, becomes very obscure, while many types of animals still boldly occupy the stage. At the earliest period from which plant remains are well preserved and plentiful, in the Devonian rocks, many of the great plant groups are fully developed, the vegetation displaying an abundance and luxuriance comparable to that of the present day. Seaweeds (Algæ), Horsetails (Equisetales), Ferns (Filicales), Club-mosses (Lycopodiales), fill the waters or clothe the land, and Seed Plants are already abundant in the form of the fern-like Pteridosperms, long since extinct. Both as regards adaptation to environment and internal structure a very high degree of specialization has already been obtained. “If a botanist,” writes D. H. Scott, “were set to examine, without prejudice, the structure of those Devonian plants which have come down to us in a fit state for such investigation, it would probably never occur to him that they were any simpler than plants of the present day; he would find them different in many ways, but about on the same general level of organization.”

In the succeeding Carboniferous Period conditions appear to have been peculiarly suitable for vegetable life, as well as for its preservation in a fossil condition. In the warm, moist climate of those times, many of the races of plants above mentioned attained an imposing size, luxuriance, abundance, and variety; and their remains, fortunately well preserved owing to conditions favourable to slow decomposition, not only furnish a rich heritage for the botanist, but supply the coal, on the energy derived from which our whole modern civilization is built up.

Before the end of the Palæozoic Period the Conifers had appeared, descended possibly from the extinct _Cordaiteæ_. With the advent of the Secondary or Mesozoic epoch the group of the Cycads, to which our modern Screw-pines belong, rose to great importance, descended probably from the Pteridosperms, and long continued to be a dominant feature of terrestrial vegetation. And then at last in the Lower Cretaceous rocks the Angiosperms, or “Flowering Plants” _par excellence_, both Dicotyledons and Monocotyledons, put in an appearance. It seems probable that they were evolved from Cycads, such as the _Bennettiteæ_, recent researches on magnificent fossil material discovered in America showing striking analogies between certain Cycadaceous flowers and those of such plants as Magnolias, Water Lilies, and Buttercups. Once established, the Angiosperms rose to primary importance in an extraordinarily short time--very possibly owing to the “invention” of insect pollination, which may have arisen at that period. In Upper Cretaceous times the two great groups into which the Angiosperms still fall, the Dicotyledons and Monocotyledons, fairly dominated the flora of the world, as they do at present. Already many types familiar at the present day had appeared, and the woods were filled with Birches, Beeches, Oaks, Planes, Maples, Hollies, Ivies, as they are nowadays.

The record of the rocks during these long periods of time contains not only the story of the rise of the great divisions of the vegetable world, but also of the decline of most of them. A few, like the Pteridosperms and the Sphenophylls, died out completely long ago; but most of the great groups of early days, such as Cycads, Ferns, Horsetails, and Club-mosses, still survive, though shorn of much of their glory.

Races which once formed vast and lofty forests are now represented by a few lowly herbs; and it is difficult to recognize in the tiny _Selaginella_ of our moors the representative of the gigantic Club-mosses of Carboniferous days. But certain plants still living retain to a great extent the features of their ancestors of the ancient rocks. One of the most interesting of these is the Maidenhair Tree (_Ginkgo biloba_), well known as a sacred tree in the East, and apparently preserved to us through the last few thousand years owing to this custom, as it does not seem to exist now in a wild state. The genus _Ginkgo_ runs back to the beginning of the Mesozoic Period, and its near relatives go back much farther still to the Devonian; the group to which it belongs, _Ginkgoaceæ_ (probably descended from the _Cordaiteæ_), attains its maximum in the Jurassic, the “Age of Reptiles,” and the existing species or its near relatives saw the Earth teeming with fantastic Saurians, including huge brutes, longer than the greatest whale, which browsed on trees or devoured creatures scarcely less terrible than themselves, while others of different form, occupied the sea, and others again of nightmare appearance dashed bat-like through the air. This solitary representative of a great and ancient race is of quite peculiar interest in that it is the highest plant in which is preserved the primitive feature of fertilization by the medium of water, the male cell being endowed with the power of motion, and reaching the egg-cell by means of swimming.

Throughout the Tertiary or Cainozoic Period the dominance of the Angiosperms became more pronounced, and already in the Eocene a flora flourished much resembling in a general way that which now occupies the Earth. Long periods succeeded the Eocene, of which the record is poor so far as plant remains are concerned, at least as regards these countries, but no further great botanical revolutions took place. Through the Miocene Period, with its luxuriant evergreen, subtropical vegetation, we are led to the Pliocene. During this period the climate once again cooled down, and towards the end of it, under conditions very like those prevailing in England at present, many of our familiar species of wild flowers and trees at length made their appearance--Marsh Marigold (_Caltha palustris_), Sloe (_Prunus spinosa_), Blackberry (_Rubus fruticosus_), Hawthorn (_Cratægus Oxyacantha_), Cow Parsnep (_Heracleum Sphondylium_), Bogbean (_Menyanthes trifoliata_), Gypsywort (_Lycopus europæus_), Sheep’s Sorrel (_Rumex Acetosella_), Birch (_Betula alba_), Hazel (_Corylus Avellana_), Oak (_Quercus Robur_), Yew (_Taxus baccata_), Bur-reed (_Sparganium erectum_), Cotton-grass (_Eriophorum polystachion_), Royal Fern (_Osmunda regalis_). The remains of these occur in the “Cromer Forest-bed,” a series of estuarine deposits--laid down perhaps by the ancient Rhine--which underlies the boulder-clay cliffs of the Norfolk coast, and forms almost the only plant-bearing beds of Pliocene Age found in the present land area which we call Britain.

And now, just as a point is reached when at length we think we shall see our present British flora emerging fully from the obscurity of the ages, a dramatic interruption occurs, which confuses the record and brings us into difficulties of many sorts, giving rise to controversies which are still far from being settled. The climate becomes suddenly colder, and Europe is plunged into the rigours of the Ice Age. Ice Ages there had been before in the long history of the world. Rocks of late Permian or early Carboniferous times bear ample witness to the existence of great ice sheets extending over wide areas in several continents where temperate or warm conditions now prevail: and puzzling deposits of later age--Cretaceous, Eocene, Miocene--have been interpreted by some geologists as the relics of subsequent Glacial Periods. But these are only distant echoes as compared with the Quaternary Ice Age, from the effects of which our country and its fauna and flora are still in process of recovery. At the close of the Pliocene Period, then, snow began to extend on the higher grounds, and glaciers to fill the mountain valleys; these conditions were intensified until all Northern Europe, including the British Isles as far south as the Thames valley, lay under a mantle of ice. The plants which occupied the ground were forced southward as the ice advanced, or exterminated by the increasing cold. After long fluctuations of climate, the extent of which appears still in doubt, the ice at length slowly passed away, leaving the surface of our country greatly altered. The ancient soils which had been in process of accumulation since last the land rose above sea-level were swept away, the surface was strewn with materials formed by the grinding down of the hills or the pushing up of sea-bottom material, valleys were choked, rivers diverted, lakes formed by dams of glacial detritus, or by the scooping action of the ice; the whole surface of the country was remodelled on new lines. Into this new land the plants remigrated, and we now view on our hills and plains the results of this repopulation. The difficulties of which I have spoken arise especially in connection with the manner of this recolonization. On a continental area one can conceive of a gradual retirement of the flora before the advance of the ice, and its subsequent remigration northward into its old haunts as the ice retired. But on an insular area like Great Britain no such line of retreat was open. The ice-free area of Southern England and possibly Southern Ireland does not appear adequate to harbour the crowd of refugees throughout the cold period. There is good evidence that the time of maximum glaciation was also one of elevation of the land, and possibly this persisted for a while after the passing away of the ice. If this were so, some relief from the congestion might have been afforded to the refugees during the cold period, and an opportunity might have existed when the ice passed away for recolonization across a land surface from the east, since a comparatively small elevation would connect the British Islands with the Continent. But that such an elevation continued for long after the passing of the ice is by no means certain. On the whole, the evidence of general glaciation of our islands as interpreted by geologists almost postulates the extinction within our area of the whole existing flora and fauna, and consequently its reconstruction by immigration when a temperate climate returned. But there is a body of evidence to be drawn from the present and past distribution of the existing plants and animals which is of great importance in this connection. Is this biological testimony in favour of the theory of the immigration of our flora and fauna during the relatively short period which has elapsed since the passing of the ice? To this question different observers have given very different answers. In order to form an idea of the nature of the problem--it is possible here to deal only with the case of the plants--we need to study briefly the composition of the present flora, from the point of view of its origin.

In the first place, it must be recalled that the British Isles are situated on a broad shelf which extends into the Atlantic on the western edge of Europe. In comparison with the depth of the adjoining ocean, this shelf is but little below sea-level, and a slight elevation of the land--much smaller than those which have occurred over and over again in recent geological times--would join our islands to Germany, Holland, Belgium, and France. The British Isles are geographically and biologically by no means a separate area, and they have derived their population, both plant and animal, by immigration at various periods of time from the great land area to the eastward. Our present flora proves the truth of this as a general assertion; a study of its constituents shows that it is essentially a reduced continental European flora. As we step from France across to England we lose a number of plants familiar on the French side. As we step again from England into Ireland a further number of plants disappears; and these losses are no doubt due either to an unsuitability of climate on the insular areas, especially the absence of a hot summer, or to the inability of the plants to cross the barriers of sea which have now existed for some time. If the whole of the flora fitted in with this idea of mere reduction of the Continental flora by elimination, the problem would be much simplified. But there are other elements in it which do not harmonize with this conception of simply a general western migration, and which give rise to very interesting problems.

Let us first consider the main mass of our flora, which is closely akin to that of the adjoining parts of the Continent. When we say that it represents a reduced Continental flora we do not imply that it is therefore uniform in its composition throughout the British Isles. We know, on the contrary, by everyday observation, that it varies much in its constituents. The principal general change is noticed if one travels from the south of England to the north of Scotland. Great Britain extends in this direction for 700 miles--far enough to allow climate to have a marked effect as between its extremities. The flora of Hampshire is very different from that of Caithness or the Orkneys. But both represent in the main the vegetation of that part of the Continent which lies in the same latitude, the Hampshire flora being akin to that of Northern France, the Caithness flora to that of Southern Scandinavia. The likeness is in each case heightened by the fact that the rocks of the respective areas correspond, producing similar soils, which tend to support similar floras. The soft Secondary and Tertiary deposits of Southern England are repeated in the Paris basin and surrounding area, while the ancient gneisses of Scotland are akin to those of Norway. To quote a few instances of this north and south difference coupled with east and west similarity: the Small-flowered Crowfoot (_Ranunculus parviflorus_), White Bryony (_Bryonia dioica_), Water Violet (_Hottonia palustris_), Yellow-wort (_Blackstonia perfoliata_), and Black Bryony (_Tamus communis_), all widely spread throughout England and Wales, die out in or about the Lake District, and are absent from Scotland; the Scale Fern (_Ceterach officinarum_) gets farther north--about half-way up Scotland--before it disappears; other plants again, widespread in the south, die out before the Mersey-Humber line, or even the Severn-Thames line, is crossed. On the Continent, the plants enumerated are mainly southern in their range. All occur widely in Central and Southern Europe, but from Scandinavia most are absent, and the rest are rare. On the other hand, some characteristic Scottish species cease as we come southward--the little _Primula scotica_, for instance, is confined to the northern extremity of Scotland; the Chickweed Wintergreen (_Trientalis europæa_) ranges only as far south as Yorkshire; and the beautiful Globe Flower (_Trollius europæus_), so characteristic of northern pastures, creeps southward as far as the Severn. The first of these is on the Continent confined to Scandinavia; the others, though found in France, etc., are characteristic of the hilly regions there, and are much more abundant farther northward.

Next to this north-and-south change, due to climate, we may notice an east-and-west change, due partly to climate, but more perhaps to elimination, for in passing from France to Ireland we have to cross two barriers of sea. The climatic change is not unlike that experienced in going from south to north. We leave a dry climate (rainfall under 25 inches at year) for one of increasing wetness, a warm for a cool summer, a colder for a milder winter.

The chief difference between the extreme west of the British Isles and the extreme north lies in the warmer winter of the former, frost being almost unknown in the milder spots. But the general similarity of northern and western conditions as opposed to eastern and southern leads to a fusing of the northern and western plant groups, so that on a map designed to show the distribution of our species analyzed according to their general range in Europe, the grouping of plants in the British Isles will be found to be roughly north-western as opposed to south-eastern. The further change due to elimination of species has been already referred to. Most plants no doubt have spread in our islands as far as prevailing climatic and soil conditions allow, but in other cases the sea-barriers seem to have put a period to their natural advance. Considering the wide range of conditions of climate and soil under which, for instance, the Hairy Crowfoot (_Ranunculus sardous_), the Common Rock-rose (_Helianthemum Chamæcistus_), the Needle Furze (_Genista anglica_), and the Small Marsh Valerian (_Valeriana dioica_), occur in England, Wales, and Scotland, it is difficult to impute their absence from Ireland to climate.

Thirdly, we find (as we have already seen in the first chapter) varying conditions of soil intruding themselves and producing such local changes in the grouping of the plants as may quite obscure the broader differences just dealt with. Were our islands a plain formed of uniform materials, the gradual changes from south to north or east to west might be traced step by step. But their surface is most diversified; their rocks contain an epitome of the whole geology of Europe; the soils are consequently various: from the point of view of the plant world the area is an archipelago: for some plants a desert with occasional oases, for others an oasis enclosing occasional deserts. Certain species are confined to the Chalk--for instance, the Box (_Buxus sempervirens_) and the Stinking Hellebore (_Helleborus fœtidus_)--while to others a limy soil is a barrier comparable to that formed by the English Channel. It will be seen, then, that when we speak of the flora in general being a reduced Continental one, many considerations, geographical, climatic, and edaphic, must be duly taken into consideration if we are to understand the composition and distribution of our vegetation.

But making all allowance for these various disturbing influences, there are found in our flora certain plant groups which will not fit in with this general conception of immigration from the east. Let us take a few examples. In fir woods in Dorset, until some forty years ago (when it was exterminated), grew a slender little plant allied to the Lilies, too little known to have a popular English name, and called by botanists _Simethis planifolia_ or _S. bicolor_, the latter name having reference to the fact that the flower is purple on the outside, white on the inside. This plant is unknown elsewhere in Great Britain, and was at first set down by H. C. Watson, the leading British plant geographer, as an alien or denizen, not a true native; but the fact that it grows over a considerable area of very wild ground in Kerry (its only Irish station), far from possible sources of introduction, and undoubtedly native, indicates a strong probability of the plant’s having been indigenous in Dorset also. It is not present on the adjoining parts of the Continent, but turns up again in the Pyrenean region, some 500 miles to the southward, and may be traced thence into Italy and North Africa. Did this instance of an apparent migration from the south stand alone, it might not excite much attention, and we should probably be inclined to attribute the plant’s peculiar and discontinuous distribution to the extinction, perhaps by human agency, of intermediate stations. But it stands by no means alone. In Cornwall two

pretty Heaths (_Erica vagans_ and _E. ciliaris_) are found, the latter spreading to Dorset. They occur in no other stations in the British Islands, and elsewhere only in the Pyrenean region. North Devon is the only home in Great Britain for the handsome Irish Spurge (_Euphorbia hiberna_), which in Ireland is distributed along the west and south coasts, being very abundant in Kerry. Outside the British Isles it also is confined to the Pyrenean area. Crossing into Ireland, we find along the south and west coasts no less than seven plants unknown in Great Britain, and elsewhere found only or mainly in the Pyrenees. Of these, three Heaths (_Erica mediterranea_, _E. Mackayi_, _Dabœcia polifolia_) are confined to Connemara and the Pyrenees; two Saxifrages, the London Pride (_S. umbrosa_) and the Kidney-leaved (_S. Geum_), with their Irish headquarters in Kerry, are likewise confined to the Pyrenean region. The beautiful Large-flowered Butterwort (_Pinguicula grandiflora_, Fig. 28), abundant in parts of Kerry and Cork, grows in South-west Europe and the Alps; while the Strawberry-tree (_Arbutus Unedo_, Fig. 29), so pleasing and unique a feature of the Killarney woods, ranges all along the Mediterranean. A little Orchid, _Neotinea intacta_, found on limy soils in Galway and the adjoining counties, and a Grass (_Schlerochloa festuciformis_) which occurs on sheltered shores on both the east and west sides of Ireland, are likewise confined elsewhere to the Mediterranean region. So it will be seen that along the south-western and western borders of the British Isles there is scattered a well-marked group of plants belonging to the Pyrenean and Mediterranean floras, whose English or Irish stations are quite discontinuous with their nearest Continental habitats. Here clearly is something which calls for explanation; but before discussing the question attention may be drawn to a still more remarkable plant group of our western coasts, which mingles with the southern group referred to.

In damp meadows all round Lough Neagh, in the North of Ireland, grows an Orchid, _Spiranthes Romanzoffiana_ (Fig. 30), whose greenish-white flowers possess a delicious fragrance resembling that of its ally, _S. spiralis_, the Autumnal Lady’s Tresses. _S. Romanzoffiana_ occurs also in Co. Cork, but we may search in vain for it throughout the rest of Europe. It is an American plant, widely spread throughout Canada and the northern States, and found on the Asiatic as well as the Alaskan side of Behring Sea. Again, in pools along the western Irish coast from Cork to Donegal, and also in the Hebrides, grows the Pipewort (_Eriocaulon articulatum_), a little aquatic with a tuft of grassy leaves from which a slender stem rises above the water, bearing a button-like head of small grey flowers. This plant also is absent from all the rest of Europe and from Asia, but widely spread in northern North America. The little Blue-eyed Grass of Canada (_Sisyrinchium angustifolium_), again, grows abundantly in many areas in the West of Ireland, where it would seem to be undoubtedly native, and is otherwise confined to North America. One or two other plants, of the same foreign distribution, have in Europe a less restricted range; they need not be mentioned individually, for enough has been said to show that along the western coasts of the British Isles

there is a small but well-marked element in the flora which has its home in the northern portion of the New World; in our islands these species live side by side with the Pyrenean and Mediterranean plants lately dealt with. Here, then, is the problem set before us. How are we to account for the presence of these unexpected strangers in a flora derived in the main from a westward migration from the adjoining parts of the Continent, from which they are absent? And especially what are their relations to the Glacial Epoch, during which the Continental flora was forced far southward by the advance of the ice, while that of our own islands was probably greatly reduced, and the balance forced into limited refuges in the south-west, if it survived at all? It should at once be pointed out that these peculiar Pyrenean and American elements in our flora are matched by similar elements in the fauna. Into the zoological evidence we cannot go here, but one well-marked species of each geographical group may be mentioned. The Spotted Slug of Kerry (_Geomalacus maculosus_) is elsewhere confined to Portugal; while a little fresh-water Sponge, _Heteromeyenia ryderi_, widely spread in Irish lakes and rivers, and occurring also in Scotland, is otherwise exclusively American. In speculating, therefore, as to the origin of the plants, we must not leave out of account the question of the corresponding animals.

First of all, is it possible that these unexpected organisms were introduced into our islands by man? In an earlier chapter it has been seen how human trade and intercourse have imported into our flora plants from the uttermost ends of the Earth. May we seek in this direction an explanation? The evidence is entirely against such a solution. These plants (and animals) are found chiefly--many of them entirely--in the wildest parts of the country, and bear fully the stamp of natives of old standing. Human foreign intercourse is not so old but that the introductions which it effected are still easily discernible to the student: the plants which have come to us thus bear the imprint of their origin; they spread outwards from centres of human activity, and are absent from undisturbed areas; they cannot in most cases compete with the indigenous vegetation, and only exist by confining their attempts at colonization to places where man has ousted the native flora--such as tilled land, roadsides, railway tracks. Even those aliens which have succeeded in winning a place among the native plants, such as the Monkey Flower (_Mimulus Langsdorfii_) or Michaelmas Daisies (_Aster_ spp.) of North America, which are found sometimes in quite wild situations, the experienced field botanist detects readily enough. The introduction of the plants in question by man has never been advocated by a responsible biologist.

Assuming, then, that these groups owe their presence to natural agencies, the next question that arises is, Could they have come to our shores across the existing seas, or must we relegate their arrival to periods when different distribution of sea and land would aid their migration by allowing them to travel across a land surface, or at least to cross sea-barriers less wide than the present? This leads us to consider the means of dispersal possessed by the species in question, and to measure these against the nature of the barriers they would have been called on to cross. An investigation on these lines would be lengthy, and out of place here. The reader has already from Chapter III. acquired some insight into the powers as well as the limitations possessed by seeds for crossing such barriers. Summing up the evidence briefly, it may be said that the seeds of none of the southern group float in water; consequently transport by currents is ruled out. Secondly, none of them is so light (see pp. 62-69) as to render it possible for them to cross the intervening sea by wind currents; very much the lightest seeds in the group are those of the Orchid _Neotinea intacta_, yet even these could not on any reasonable theory have been transported by wind from the plant’s nearest station (in Southern France); the high speed of fall of the small seeds of the Pyrenean Heaths or Saxifrages renders their wind transport, even from the smaller distance which has to be reckoned with, in their case still more improbable. There is left, then, the agency of birds (see p. 70): can we look to these swift messengers for assistance? The rapid digestion of birds renders it futile to expect that even those which do not crush the seeds which they eat could bring over from the Pyrenees seeds which they have swallowed; so we are forced back on the uncertain method of ectozoic dispersal: that is, on the assumption that seeds of these plants have been imported by becoming entangled in the feathers of birds, or by adhering--possibly with the aid of mud--to their feet. That seeds are transported by these means has been shown by the observations of Darwin and other observers; but that the seeds of a number of different plants, growing in different situations, should be brought thus from the Pyrenees and Mediterranean to our western coasts is a highly speculative suggestion. If we discard it, there is left the hypothesis that the plants migrated long ago overland, at a time when the western coastline of Europe was continuous and lay farther seaward. Such conditions have not occurred since the Ice Age; so we have to assume that the plants, arriving perhaps in Pliocene times by slow terrestrial dispersal, and subsequently cut off by invasions of the sea upon their line of advance, survived the cold and ice of the Glacial Period within the limits of our islands. That appears, on consideration of the geological evidence of widespread glaciation, sufficiently improbable; but we must remember that the evidence supplied by the plants is buttressed formidably by that of the corresponding animals, some of which, such as the Kerry Slug, are far less fitted for transmarine dispersal than are the seeds of plants. Also, we are faced with the problem of the American plants, and such organisms as the American Sponge, _Heteromeyenia_: a direct crossing of the ocean appears for them wholly impossible. Yet if they crossed over long-gone land surfaces, their arrival on this side of the Atlantic must be very ancient, and they must certainly have weathered successfully the Great Ice Age. The problem, it is clear, is an exceedingly difficult one, upon which it would be rash to pronounce any hasty opinion. Students of the subject have come to widely difficult conclusions: some holding with Edward Forbes that these Lusitanian and American organisms represent the very oldest element in our fauna and flora, having migrated over bygone land surfaces in distant times and successfully survived the terrors of the Glacial Period; others claiming a much less remote period for their immigration. Indeed, one eminent recent writer on the subject, the late Clement Reid, considered that the Lusitanian plants are among the most recent arrivals in the country, their introduction being due mainly to birds driven by exceptional gales.

The question of the Lusitanian and American elements in our flora has been treated at some length both because it offers one of the most interesting problems in British botany, and because it affords a good illustration of the far-reaching nature of the questions which may lie behind the occurrence on our hills or in our valleys of even the humblest plant or animal. Each organism has a long record behind it, stretching far beyond the earliest periods of human history; and it is only by wide and patient study that we can hope to trace any portion of its story.