Fig. 10.—Diagram to illustrate a cyclone travelling towards the east. The two concentric circles represent isobars, that is, they are lines drawn through points where the barometer registers the same (low) pressure. Into the area of low pressure so formed the winds blow strongly in the direction known as counterclockwise, and are represented by the arrows whose double barbs signify their strength. It will be noted that in the rear of the cyclone the winds are northerly. They thus chill the air here, and by chilling itraisethe pressure. The winds to the front of the cyclone are warm because southerly, they therefore tend tolowerthe pressure here by warming the air, and the result is that the isobars tend to be displaced towards the east, and at the same time become deformed. In other words, the cyclone moves to the east.
We may take British weather, which has become a proverb on account of its variableness, as a text for a brief discussion of the subject.
The daily variations in our weather, as all who have read weather reports know, are chiefly determined by the movements of areas of low pressure or cyclones, which mostly come to us from the Atlantic, and continue eastwards past us, often towards the Baltic. We have already noted the occurrence of what we have called a permanent area of low pressure in the North Atlantic, but this “permanent area” in point of fact is due chiefly to the constant passage here of cyclones, or moving areas of low pressure.
The causes of the eastward displacement of these depressions are interesting. One cause is the general eastward movement of the atmosphere in this region, produced in the fashion already described. This movement necessarily raises the pressure to the west of the depression, owing to the influx of fresh air, while the onward movement of the air in front of the depression lowers the pressure there, and so produces displacement. Again, the air is sucked into a depression in the direction opposite to the hands of a clock, and a moment’s reflection will show that this means that the winds to the east of the depression are southerly and those to the west ofit northerly. The warm southerly winds in front tend to lower the pressure by warming the air, while the cold northerly winds behind it raise the pressure by cooling the air. This again produces a displacement of the depression towards the east (seefig. 10).
The fact just described has an interesting practical result. If after a day or night of storm and rain, the temperature falls, we know that the depression causing the storm has passed us, and that we are feeling the effects of the colder winds in its rear. If the thermometer suddenly rises again, then a new depression is approaching, and we are feeling its warm breath before its winds reach us. The clearness and chilliness of the air after a stormy or windy period gives us one of our commonest meteorological sensations, and produces a marked psychical effect, reflected in much of our literature.
One other reason for the eastward motion of the cyclones with us is that they seem to prefer damp air, and so tend to follow the North Sea and pass towards the Baltic, where they often die away.
In the British area, though the depressions move faster in winter than in summer, theyhave only a mean speed of about 16 miles an hour, while in the United States their mean speed is 25 miles per hour, and their effects are often disastrous except when discounted by the warnings of the Weather Bureau.
In the case of the British Isles cyclones are most frequent and best marked in winter, and they are of great importance in producing our mild and windy winters. In summer they travel further northwards, and as a ruleaffectour climate less. When, however, from causes still inadequately known, they are better marked in summer than usual, we have a “bad” summer, that is, one which is wet and relatively windy.
The fact that the English Channel is one of the favourite tracks of cyclones has been an important element in protecting the British Islands from foreign invasion, while we all know that it is also a factor in diminishing free intercourse with the Continent.
Fig. 11.—Diagram showing the changes in temperature, pressure and wind due to a cyclone passing to the north of a point of observation A. The passage of the cyclone figured occupied a period of six days. It will be noted that as it approaches A the wind is southerly and light (arrows with single barbs) and the temperature high. As it passes the winds become violent (arrows with double barbs), and shift to the south-west, and the barometer falls rapidly. As it disappears the pressure rises, the temperature falls, and the wind veers to the north-west, while remaining violent. The fall of the wind and its shifting to a south-westerly direction mark the return to the normal condition of affairs, the influence of the cyclone being past.
The second point of importance about our weather is the periodic occurrence at some part of our area of anticyclones, or areas of high pressure, out of which the winds stream gently in the same direction as the hands ofthe clock. These areas of high pressure do not display the same tendency to move as do the cyclones, and are most frequently merely displaced by advancing cyclones. For reasons into which space does not permit us to go fully here, anticyclones have a very different effect in summer and in winter. In winter they may bring to us the continental cold, and make our weather abnormally severe, though often bright and fine. On the other hand, in summer they bring to us continental warmth, so that “good” summers are those in which anticyclonic conditions are most frequent, while “severe” winters are due to the same cause. Anticyclones also sometimes induce a curious form of inversion, in that places to the north of a given spot may have temporarily a higher temperature than places to the south. It is such facts which are entirely masked by “mean” figures.
We do not as yet understand the causes which make cyclones sometimes more numerous or better marked than usual, which cause them sometimes to cross our area, and at other times to travel too far north or too far south to influence our weather. It is possible that further investigation in the future mayunravel this problem; it is practically certain that a freer use of wireless telegraphy, and the establishing of meteorological stations in northern seas, would give weather forecasting a definiteness and accuracy which it does not yet possess.
Fig. 12.—British weather map for Nov. 29, 1910. A cyclone lies over the south of Scandinavia, and into it the winds are sweeping strongly in a counterclockwise direction. An anticyclone lies over Iceland, and from it the winds are streaming gently in a clockwise direction.
We cannot follow this interesting subjectfurther here, but we have said enough to illustrate its geographical significance. As a science or sub-science by itself it will form the subject of a special volume in this series. It may be enough to point out that theDaily Weather Report, published by the Meteorological Office at a cost of one penny, and reproduced in some daily newspapers, is a document well worth the careful study of those with any interest in geography.
We have now taken a general survey of the earth’s surface, have noted its mountain heights and its ocean depths, watched the formation of hills and valleys which is due to the joint action of atmospheric agents, running water and ice, and considered briefly some of the points of interest about climate. We next pass to that most characteristic feature of the surface, its clothing of plants. Except where the surface of the ground has been artificially sterilised by man, or is rendered unproductive by ice, by lava, by a total lack of water, or by the existence of poisonous salts, it is clothed with vegetation, and it is the presence of this vegetation which is its most obvious character.
Here, however, as in other regions of thought, the geographical standpoint has only been reached slowly. Man’s habit of analysis made him study grasses and trees for long generations before he got back to the forest and to the grassland as they occur in nature. Plants as individuals are the province of the botanist, but those plant groups which are the expression of the interaction of climatic factors, soil, and surface relief, are the concern of the geographer.
When we take a general survey of the face of the earth from the point of view of plant geography, we note three main conditions. In certain regions, alike in the tropics and in temperate zones, we find that plants reach their maximum size, combined with great differentiation of structure, and the formation of woody stems which offer great resistance to varying conditions of climate and weather. Such highly-organised plants form forests, which still dominate over a large part of the earth’s surface.
Man’s nearest allies, the anthropoid apes and the monkeys, are for the most part forest animals, and the lowest races of men are still forest dwellers. Where man is a forest dweller he seems not to reach his full size,as we see in the case of the pigmies of the Congo forest, and the negritos of the Philippines, and he suffers from a chronic insufficiency of food, which acts as a check both to his mental and physical development. There has, therefore, always been war between evolving man and the giants of the plant world, a war which has swept the forests away from many of the more civilised parts of the globe, and which still continues, though man’s victory is now so complete that he can afford to be generous, and give protection to the remnants of his former foe.
But over parts of the globe the climate, and especially the amount or distribution of the rainfall, makes it difficult or impossible for forests to grow naturally. Here other types of plants, lower in stature, and evading rather than facing the problems of winter cold or summer drought, flourish and form what we call the grasslands. The grasslands favour man in several respects. They feed the animals upon which he depends for food, for clothing, and for the conveyance of his person or property, and they offer much less resistance than the forest to his agricultural operations. Even the large herbivorous mammals which in their wild state haunt theforests, usually leave these at night to feed upon the grasslands, so that it is the grasslands which have largely fed man at every stage of civilisation. The atmospheric conditions within the woodlands also, the deficient sunlight, the humidity, and so forth, seem unfavourable to human development.
Where the conditions are especially unfavourable to plant life, we find that even the grassland plants are unable to keep up the struggle, and diminish in number, losing their power of forming a complete covering for the soil, and thus the grassland passes into desert, whether the hot waterless desert of low latitudes, or the cold frozen desert of northern ones.
In the most general sense, then, we may say that these three formations, woodland, grassland and desert, divide the surface of the land among them, and between them there is constant conflict. The grasslands are for ever attempting to encroach upon the woodlands, and in this attempt they have been assisted, sometimes to too great an extent, by the operations of man. Similarly the desert is always striving to encroach upon the grassland, and in this endeavour it has been sometimes involuntarily aided by man,who has also done much voluntarily to reclaim the desert land for the grasses.
Let us note next the particular conditions which favour woodland, grassland and desert respectively. The distribution of plants over the surface of the earth at large is determined by a number of factors, by the amount of heat, by the amount and distribution of precipitation, by the nature and strength of the winds, by the characters of the soil, and so on. But forests occur under the equator and also far to the north; we have cold deserts as well as hot ones; there are extensive grasslands in the Sudan as well as in the Canadian Far West. This proves that the varying amounts of heat may be neglected in considering the cause of the distribution of the three great plant formations.
Again, the soil is of minor importance, for different types of forest and of grassland occur on different types of soils. We are thus led to the conclusion that it is the precipitation and the wind which determine the distribution. To understand the reason for this we must consider the needs of different types of plants in the matter of water.
Plants can only take in the mineral constituentsof their food in the form of a solution, and this solution must be weak, or it has a poisonous effect. For example, sulphate of ammonia is a valuable manure, but if a considerable amount be dissolved in water and applied to the roots of a growing plant, death may very likely take place.
It is a necessary consequence of the fact that plants can only absorb weak solutions of their food salts, that their roots take in more water than is actually needed by the plant.Oneof the functions of the leaves is therefore to get rid of surplus water, the process being called transpiration. Transpiration takes place faster in a tall plant like a tree, which grows up into dry layers of the air, than in a low plant like a grass. It takes place faster in windy weather than in calm. Other things being equal it takes place faster in warm weather than in cold, and the larger the plant and the more numerous its leaves the more water is given off, that is, the more water is returned to the air from the soil.
The result of all this is that forest trees require far more water than grassland. It has been calculated that a beech wood aged 50 to 60 years gives off during the growingseason 354 tons of water per acre, which illustrates the drying effect of the presence of the wood. Similarly, the effect of tree-planting in the marshy regions of France and Italy, where the soil as a consequence has dried and the marshes disappeared, shows how great a demand upon ground water trees make, as compared with grasses and low growing herbs.
On the other hand, although trees take an enormous amount of water from the soil, they can draw their supplies from a large area. It is the extremities of the fine branches of the roots which take in the water, and these pass deep down into the soil, and spread out over a vast area. In other words, trees avail themselves of the water in the deeper layers of the soil, and can tolerate relatively long periods of drought, if the surface drying of the soil does not extend to the deeper layers. In hot summer weather grasslands brown and wither long before the trees show any signs of water-famine.
In consequence, we may say that as long as the total rainfall of a region is sufficient to ensure a constant supply of moisture in the subsoil during the growing season, trees can thrive, even if little or no rain fallsduring this season. On the other hand, drying winds are very hurtful to trees, especially if they occur at a period when the tree is unable, either because of the coldness of the subsoil, or because of its dryness, to take in fresh water to replace that which is lost. The hurtfulness of late frosts is largely due to the cold suddenly checking root absorption at a time when the growing parts, acted upon by the spring winds, are giving out water freely.
Grasses transpire less freely than trees, but their root system is much shallower and less well-developed. They depend upon the water in the upper layers of soil, and must have frequent, even if gentle, showers during their growing season, while they are quite indifferent to drought and even to cutting winds during their resting period.
A little reflection will show that it results from these facts that woodland, grassland and desert do not form a continuous series. It may quite well be that woodland passes through scrub into desert without the intervention of grassland. Right across Europe there is (or was) a broad belt of forest. Southward towards the Mediterranean this thins out into a characteristic form of scrub, calledmaquis, whose characters we shall describe later, and this scrub passes in all directions into desert land. Here no belt of grassland intervenes, for the rainless Mediterranean summer makes the growth of grass virtually impossible, except where special conditions,e. g.hills, introduce modifications. Contrasted with this we have the conditions in North America where,e. g.in Canada, the western coast is densely forest-clad, as is also the eastern region. In journeying eastward after crossing the Rocky Mountains the forest dies away into grassland, and the same thing happens, though more slowly, in a westward journey. The reason is that in this case there is a steady diminution of precipitation on passing to the interior, but what precipitation remains is, as we have seen, largely, though not wholly, summer rain, and is, therefore, sufficient to determine the growth of grass, though not of trees.
Again, in North Africa the forests of the Atlas Mountains pass directly, without intervening grassland, into the Sahara desert, but to the south of the desert the grassy and park-like Sudan separates the desert from the luxuriant tropical forest. In the lattercase, however, it is possible that man’s influence has counted for something.
On mountains, in whatever latitude, the conditions are much more uniform, partly because it is wind, assisted by temperature variations, which is the dominating factor. Moisture is usually abundant, but high up what is called physiological drought occurs; that is, the temperature is too low for the plants to be able to absorb the abundant water.
In ascending any mountain, the following are the chief changes which occur. The lower slopes will probably be cultivated. As we ascend the precipitation increases, and forests appear. First we have probably a belt of deciduous trees, passing above into the more resistant conifers. This belt usually ascends higher on the south than on the north side, and higher on mountains which occur in a group than on isolated peaks. As the wind is more and more felt, and increases the dangerous transpiration of winter the trees become more and more dwarfed to escape its force. There may be a belt of prostrate mountain pines above, marking the tree limit; in any case the trees are gradually replaced by dwarfed shrubs. Then comesthe zone of Alpine plants, the grasses making a complete sward, but being accompanied by many other plants. Gradually, as the soil becomes scantier, and the surface more rocky and exposed, the continuous sward disappears, and the conditions of a cold desert appear. A few scattered plants occur, ceasing near the snow-line, the highest being usually plants of simple structure like mosses and lichens.
As we have already indicated, in the case of the mountains of Europe there are often glacial shelves at considerable elevations, whose covering of fine débris determines the growth of peculiarly fine grass. The economic value of this grassland has in many cases in the Alps induced man to destroy the forest in order to increase pasture land. The result has often been disastrous, for once the trees are cut down the forest soil is rapidly destroyed by weathering, especially on slopes, the courses of streams are altered by the more rapid run-off, and widespread flooding and destruction of pastures have sometimes resulted. In North America, similarly, man’s attempt to increase pasture land or arable land at the expense of woodland has often led to disastrous consequences.
We have already spoken of the special features of the Mediterranean climate, and indicated that its peculiarities are reflected in its vegetation; we must now consider this vegetation in a little more detail. The fact that the region is chiefly visited by the inhabitants of more northern climates in spring gives rise to a somewhat erroneous impression in regard to the plants. In spring the Mediterranean vegetation is at its best. The mild winters permit the plants which further north die down or cease to grow, to go on blooming. The rains so moisten the soil that the first warm days cause very rapid growth in those plants which finish their activities before the hot, dry summer begins. They must flower and seed in spring, and die down till the rains of autumn awaken them again.
In our own country we have a few plants which hurry through their activities in this way. The lesser celandine, the wood anemone and a few others strive to flower and fruit before the forest trees are thickly clad with leaves. The snowdrop, even the wild hyacinth, though it is much later, similarly limit their active life to a short period in spring. This phenomenon, only suggestedin our climate, is very marked in the Mediterranean area.
That region is especially characterised by its richness in bulbous and tuberous plants. These, as all who have grown hyacinths or narcissuses know, demand relatively large amounts of water during their short growing period. In spring, therefore, the shores of the Mediterranean are bright with many kinds of anemones, with narcissus, asphodel, bell hyacinth, Allium, tulips, and so on, all awakened by the spring warmth and the spring rains. Accompanying them are many bright-coloured annuals, also in a hurry to race through their life-history before the terrible drought of summer. Now also the grass grows, and the autumn-sown corn becomes tall. As the weather grows hotter and drier, the plants with bulbous and tuberous roots die down to the ground, the annuals die altogether, leaving their seeds to wait till the autumn rains before they sprout. The grasses turn brown, and the peculiar parched appearance of the Mediterranean summer spreads over the land.
To a northern visitor at this season it is not luxuriance but desolation which is the prevailing note. Except on the hill slopesthere are no masses of broad-leafed foliage trees—there is not the deep bright green characteristic of the summer woods further north. The trees do not reach a great size; the leaves are usually small, and the fact that they strive to avoid the sun by arranging themselves with the edge upwards instead of the flat surface, makes them appear smaller than they are. They are often needle-shaped, sometimes shining and coated with resin, sometimes silvery owing to a coating of hairs on the under surface. Many plants have spines or thorns, and succulent plants like agave, aloe and prickly pear are common. The absence of a complete covering of vegetation causes the surface soil to dry completely, and so form clouds of dust which adds to the generally desolate appearance. Indeed, the brown powdery appearance of the soil is one of the points which especially strikes the stranger, accustomed to the darker, moister soil of the north, always covered with vegetation, except where man has interfered.
Here and there, however, are indications that even this parched brown earth holds wealth for man. The vines, if dusty and far less luxuriant than one expects, are loadedwith ripening fruit. The gorgeous scarlet flowers of the double pomegranate gleam amid the dark foliage; the gnarled and twisted olives show on close inspection masses of small green fruits; the oleander bushes are covered with pink flowers; there are great round balls on the orange and lemon trees, and many other fruit trees are loaded with produce.
Let us sum up first what man gains from the plants of the Mediterranean, and then look at some points in regard to the wild plants. In the first place, we see that man takes advantage of the rapid growth of annuals in the early part of the year. The annuals most useful to him, here as elsewhere, are, of course, the cereals, especially wheat, which, if sown in autumn, is nourished by the winter rains, and grows rapidly with the warmth of spring to ripen in May, June or July, according to the locality.
In the second place, certain trees or shrubs, by reason of their resistance to drought, and their elaborate root system, which enables them to gather water from the deeper layers of the soil, will produce succulent fruits without needing artificial supplies of water. The most important of these, throughout thewhole Mediterranean area, are the vine and the olive. The olive supplies the oil which is all the more necessary in that the absence of grass makes pastoral industries, and therefore the production of cheese and butter difficult or impossible except in the high grounds, while the vine supplies the wine which with bread and oil form the essential parts of the diet of Mediterranean man.
The olive tree, which is indigenous, may be regarded as one of the most characteristic trees of the area, and it is interesting to note that the novice not infrequently confuses it with another tree, almost as characteristic the evergreen or holm oak. The two are not nearly related, the olive belonging to the same family as the lilac and privet, while the evergreen oak is a true oak. Both trees, however, show similar adaptations to summer drought, and their resemblance to one another is a good example of convergence due to a similar environment. Both have small evergreen leaves; small that they may not lose too much water in summer, evergreen that they may assimilate even during the winter. Both have their leaves silvery beneath, which again prevents loss of water; both have gnarled trunks, branching lowdown, in order that the leaves may avoid the dry upper layers of the air. Adaptations of this kind are present to a greater or less degree in all the trees which are tolerant of Mediterranean conditions, and many of these trees yield useful fruits.
In addition to the cultivated plants mentioned, a great number of others are grown within the area, as we shall see later, but the point of interest is that the plants which have been of importance in the history of the region have been either annuals which ripened early, or fruit-bearing trees with special adaptations to resist drought.
Apart from the annuals and the bulbous and tuberous plants already described, the wild plants are chiefly shrubs or stunted trees with similar drought-resisting characters. During the long ages he has inhabited the Mediterranean, man has doubtless contributed largely to the destruction of the forests which are now, as we have seen, represented by the stunted scrub or maquis. But on climatic grounds we cannot suppose that the Mediterranean forests had ever the luxuriance of those further north, or of the tropical forests of the south.
Where there is sufficient rain chestnutwoods occur, but this is only on the hill slopes. Above the chestnut, beech may occur, as in Sicily. The maritime pine and the Corsican pine form open woods in the damper places, and the picturesque stone pine, with its rounded head, is very characteristic. We have already mentioned the evergreen or holm oak as common, and the cork oak occurs abundantly in some places. These trees, with the cypress, must have formed the primitive forests, and they still constitute the most important forest trees of the area. The occurrence of a native palm (Chamærops) is interesting as suggesting the warmth of the climate, and even on the European shores the date palm is extensively planted, though its true home is the margin of the African and Arabian deserts.
Of the characteristic shrubs the most striking are perhaps the many species of Cistus, with large almost rose-like flowers, and leaves which attempt to adapt themselves to the climate by many different devices. Sometimes they are stiff and leathery, sometimes resinous, sometimes hairy. Many plants in the area have a coating of resin on their leaves. This, no doubt, preserves them against loss of water, but also probably protects againstgrazing animals. Goats thrive in the Mediterranean partly because of the catholicity of their taste in vegetation, and in consequence the plants have had to protect themselves against their appetite as well as against drought. Only those with some disagreeable quality, hairs, spines, resin, strong flavour, etc., could hope to protect themselves in the dry season, when grass is virtually absent. It is in consequence common to find aromatic or strongly-flavoured plants with glandular leaves; lavender, rosemary, myrtle, etc., are examples.
Other shrubby plants associated with the Mediterranean are oleander, the noble laurel, the tree heath, arbutus, many kinds of broom, and generally evergreen shrubs specially adapted to resist drought.
Let us turn from this picture to the appearance presented by Central and Northern Europe. As we have seen, the forest which once covered most of the area, except the steppe region of southern Russia, has largely disappeared, but enough remains to enable us to reconstruct the picture of the original forest.
As contrasted with the (chiefly) evergreen woodland of the Mediterranean, the forestsof the low grounds are here deciduous. In summer clothed in magnificent foliage, well adapted to give off enormous quantities of water, in winter the trees stand tall and bare, exposing nothing but their branches to the winter blasts. While the buds of Mediterranean plants have no special means of protection, the typical forest trees of Central Europe have their buds carefully sheathed in scales, clothed in hairs, or coated with resin, to keep out alike the cold and the damp of the northern winter. While the leaves of Mediterranean plants are usually small, often coated with hairs beneath, often resinous, and so on, the forest trees further north have large leaves of delicate texture, with no special protection against drought.
Again, while the luxuriant forest of the tropics includes many different species of trees, the deciduous forests of cool temperate regions contain few species, and are often pure woods, that is, consist of one dominant species, forming beech woods or oak woods, and so on. The dense shade of the beech makes undergrowth difficult or impossible, but the other woods have a complicated undergrowth of many different kinds of plants, especially pronounced in spring beforethe leaves appear on the trees. But this undergrowth never reaches the luxuriance that it does in the tropical forest, and creepers and climbing plants are few.
As we ascend from the low ground to the higher, or as we travel northwards to high latitudes, the broad-leafed deciduous forests are replaced by coniferous ones. European conifers, with the exception of the larch, are evergreen, and all are more tolerant of cold and wind than deciduous trees. Pines, spruce, fir, larch, and silver fir are the most important kinds. Both at high altitudes and in high latitudes these conifers are often accompanied by birch, which is not a cone-bearing tree, but is very tolerant of cold and wind.
To the north there comes sooner or later a limit beyond which the cold and winds make further tree growth impossible. Here we come to a tundra region, where the place of trees is taken by low-growing shrubs, with small leaves and other adaptations to ensure against excessive loss of water. It is, as it were, the reappearance of the Mediterranean type, but here the cause is, not the absence of water, but the fact that the cold makes it impossible for the roots to absorb it. A conditionof physiological drought results, and only plants well adapted to prevent undue loss of water can resist such conditions of life.
A somewhat similar type of vegetation occurs over vast areas in the more northern parts of Europe, forming the moors and heaths of much of Scotland, of parts of England and Ireland, of parts of Germany, and so on. Here the presence of peat produces conditions very unfavourable to plant life, except to certain shrubby plants such as heather and other plants of the heather family, juniper, bog myrtle, and so on, and some grasses and sedges, etc., all of which have special adaptations to life in a peaty soil. Over the large areas, therefore, covered by these heaths, trees are absent, or few, and this stunted shrubby vegetation takes their place.
Large areas of natural grassland, except for the tracts of pasture land already described in the mountain regions, are infrequent in Europe. They occur in Southern Russia and in the Hungarian plain, and form part of that great series of steppes and plains which stretches into Asia, and passes into a region of deserts.
The conditions favourable to the growth ofgrass here, instead of trees, seem to be purely climatic. Very important is the prevalence of strong cold winds during winter, which is a period of drought. The scanty rains come in early summer, which suits grasses admirably, while the total precipitation is too slight for trees. The summers are hot, and the rains cease early and give place to a period of drought, very injurious to trees, while it injures the grasses little, owing to the fact that they have had time to make their growth.
The abundant natural growth of grass makes these steppe regions well suited to the pastoral industries, which tend, as civilisation progresses, to give place to agriculture.
To sum up, we have seen that looking at Europe as a whole three great plant formations are represented. We have, first, the cool temperate forest, which once extended over the greater part of the continent, wherever the conditions were suitable. This has now largely given place to arable land. Next, we find round the Mediterranean sea, and in those great peninsulas and islands which are bathed by it, a zone of modified woodland passing into scrub, remarkable for the rapid growth of annuals in the early part of the year,and for the abundance of trees bearing useful fruits. Finally, linking Europe to temperate Asia, we have belts of steppe land, characterised by a luxuriant growth of grass in the early summer, and fitted by nature for pastoral industries, which do not thrive near the Mediterranean. Another way of putting the same facts would be to say that Europe proper is a region of temperate forest, linked to Africa by scrub land passing into desert, and to Asia by steppe land passing into desert.
The flora of North America, owing to the size of the continent, offers more resemblance to that of Asia than to Europe.
Bearing in mind what has been already said about the structure of North America—with its western mountain range and eastern uplands enclosing between them a region of moderate relief—and also what has been said in regard to its climates and to the influence of climate upon vegetation, it is relatively easy to deduce the main points in regard to the flora.
To the far north there is a treeless tundra region, quite comparable to that which occurs over vast areas in North Asia, and ona reduced scale in the northern part of the continent of Europe. Next we have a wide band of predominantly coniferous forest, which, although its species are different, yet in broad outline is entirely homologous with the coniferous forest found in northern Asia, south of the tundra region. In Canada this forest consists of spruces and larches, the species being peculiar to the continent. Mingled with the conifers are smaller numbers of the hardier deciduous trees, such as birches, poplars, and willows.
What we have already said as to the climatic differences between the eastern and western sides of continents will at once suggest that this band of forest is not likely to run directly across the continent from east to west. In point of fact it stretches from Labrador in a north-westerly direction to Alaska, leaving almost the whole of the westernseaboardto be occupied by another type. This type is the extraordinarily luxuriant and beautiful western forest, consisting for the most part of conifers. It is largely these conifers which have enriched European parks and gardens within recent years, and although it is perhaps the greatSequoia(Wellingtonia)giganteawhich has most impressedpopular imagination, it must be remembered that size and luxuriance are characteristic of many species. This western forest stretches down the western seaboard to the State of California, and, indeed, persists until increasing aridity makes forest growth impossible. Its great luxuriance, compared with the scantier forests of the Mediterranean region in Europe, is partly to be ascribed to a greater rainfall, and doubtless partly to man’s interference, for the original forests of the Mediterranean must have been largely destroyed, as the western American forests are in process of being. One must remember also that the proximity of mountain ranges to the seaboard in western North America gives a heavy rainfall, and suitable places for forest growth. The fact that the trees are predominantly coniferous gives them great resistance to the summer drought. In front of the mountain ranges the coastal plain is occupied by an evergreen scrub vegetation comparable to that of the lowlands of the Mediterranean basin.
In British Columbia, where the Cascade Range lies at no great distance from the Rocky Mountains, the western coniferous forest practically clothes the whole area fromthe coast to the main range, but further south, where the Cascade Range and its continuation the Sierra Nevada are widely separated from the main range, a dry and semi-desert region occurs, between the two, which bears a desert type of vegetation, including especially a plant related to our wormwood, called sagebrush, with cactuses in the warmer parts. Another area which is too arid to carry trees, except where local conditions raise the rainfall, extends from Texas northwards to about the latitude of Edmonton or Battleford, and lies in the “rain shadow” of the Rocky Mountains. This is the region of the Great Plains, mostly too arid to carry anything but herds of cattle, and mostly forming natural pasture, being thus analogous to the steppes of Asia.
Eastward the rainfall increases, and we pass from the area of unreclaimed pasture to the prairies, now largely laid down to wheat and other food plants. Southward the Great Plains pass into the deserts of Mexico, but northwards they are separated from the northern coniferous forest by a belt of aspen, and it is in this region that the Canadians are steadily pushing the cultivation of wheatinto the plains, wherever the local rainfall makes this possible.
So far we have left south-eastern Canada and the whole of the eastern and south-eastern States out of consideration. Speaking very broadly, we may say that all this area is clothed by a forest of mixed coniferous and broad-leaved trees which is comparable to the forest which covers the greater part of temperate Europe. But it is not to be expected that a forest which extends from the northern shores of the Gulf of St. Lawrence to the extremity of the peninsula of Florida, that is, through about 25 degrees of latitude, should be uniform throughout. In point of fact, botanists distinguish three separate zones. In south-eastern Canada and the New England states the Weymouth pine (Pinus strobus) predominates, being accompanied by limes, ashes, maples, oaks, elms, chestnuts, and so forth. Further south, and especially further west, extending to the Mississippi plains, there is a deciduous forest extraordinarily rich in species. Practically all our common genera of forest trees are represented, sometimes by very fine species, but in addition there are many genera with no European representatives.Very striking is the abundance of magnolias (whence the name of magnolia forest sometimes given to this type), and species of the laurel family, as well as of liquidambar. The magnolias and liquidambar are especially interesting, because they once occurred in Europe, their disappearance there being probably caused by the glacial period as explained onp. 78.
We have emphasised above (p. 137) the luxuriance of the forests of the west coast of the States, but it should be noticed that luxuriant as its conifers are, there is a remarkable poverty in broad-leaved forms, as compared with these eastern forests, and this even in the warmer parts of the west coast. The reason is probably the same as in the Mediterranean region in Europe. The existence of a belt of desert to the south of the present “Mediterranean” region of western America made it difficult for the trees to migrate southwards at the onset of cold conditions in the glacial period, and thus many forms, which are known to have existed in California in Tertiary times, have now completely disappeared from the region, while they persist in the eastern forests to this day.
The third type of forest which occurs in the eastern half of North America is the “rain forest” of Florida and parts of the adjacent states. Here the rainfall is abundant all the year round, with a summer maximum, and the temperature is high. There is thus no need to economise water, and where the soil permits there is a luxuriant type of forest, which recalls that of the tropics, although it is poorer. Where soil conditions are unfavourable we have pine woods, conifers throughout the eastern United States always taking advantage of conditions relatively unfavourable to the broad-leafed trees.
Thus if we follow the eastern seaboard of the United States from Labrador to Florida we pass through the following floral regions:—(1) Coniferous forest, with relatively few species, (2) mixed coniferous and deciduous forest with chiefly the harder types of deciduous trees, (3) predominantly deciduous forest with many of the larger-leafed and more delicate forms, and finally (4) forest of the sub-tropical rainy type, intermixed with coniferous woods on the barren sandy soil and in the swamps.
The western coast shows more uniformity, the western type of coniferous forest stretchingfrom Alaska to California, though it is richer, and more luxuriant in the warmer regions when moisture is still obtainable. As the moisture diminishes the forest dies away and desert or semi-desert conditions supervene.
In the last chapter we looked at a few of the interesting generalisations which have emerged of late years from the study of plant distribution. An enormous amount of detailed investigation had been done before these generalisations were arrived at, and though still much remains to be done, yet the broad lines of a science of plant distribution may now be said to be established. The scientific study of animal distribution has not yet reached a corresponding stage of advancement, partly no doubt because the dependence of the more highly organised and active animal upon the physical conditions is less close than that of the stationary plant, so that the subject is more difficult. Facts are accumulating on all sides, but the subject is still rather at the level of collecting informationthan at that of laying down broad generalisations. There are, however, indications of progress in many directions, and an attempt will be made here to suggest some of the lines along which research is especially busy at the present time.
In speaking of plants we confined our attention exclusively to land plants, for the reason that aquatic plants are usually small in size, relatively simple in structure, of somewhat limited vertical distribution, owing to their dependence upon light, and of little direct importance to man. In considering animals, on the other hand, we cannot exclude the aquatic forms, which are often of great human importance. In many regions man depends largely, sometimes even exclusively, on the animals of the sea for his food. We shall, then, begin with some account of aquatic animals, considering the subject, as before, especially from the point of view of the inhabitants of Europe and North America.
Beginning with the sea we find that the scientific study of marine animals received an enormous impetus from the work of theChallengerexpedition. The results of that expedition appeared in many large volumes, which form a conspicuous feature in anycomplete scientific library and contain a mass of useful material. TheChallengerexpedition was followed by many others, European and American, and the result is that we now know a great deal about marine animals and their distribution. Further, the Fishery Boards of various Governments carry on continuous observations on the conditions of life in the seas near their coasts, which have added and are adding enormously to our knowledge.
We cannot here consider in detail the various facts brought to light by these means. Only a few general points can be touched upon. One interesting generalisation is that the life of the ocean can be divided into three groups: the life of the littoral or shore zone, the life of the open ocean (pelagic fauna), and the life of the great ocean depths (abyssal fauna). The last, though of great zoological interest, is so remote from human life that we need not consider it. The pelagic forms include both the small delicate organisms which float passively with the ocean currents, and also powerful swimmers like many fish, and aquatic mammals such as whales and seals. The littoral forms live in the region which is within the reach of landinfluences, that is, from low-tide mark to the edge of the Continental Shelf (cf.p. 27). Among forms directly important to man they include many fish; crustaceans such as crabs and lobsters; shell-fish such as oysters, mussels, clams, etc.; less important forms such as sea-urchins, which are extensively eaten in the Mediterranean; sponges, an important article of commerce; the various corals, especially the precious coral, and so on.
Of the useful marine animals, those which are most readily captured are the littoral forms, many of which, on shores where the tides are well marked, are exposed, or at least brought within easy reach, by the daily ebb and flow of the tide, and can be obtained with the minimum of apparatus. The extensive shell-mounds found on many shores,e. g.on those of Denmark, show at how early a date man availed himself of the abundant food supply to be obtained on the shore rocks. All edible animals found in the sea are “fish” to maritime populations, but fish in the restricted sense are usually more active, and require more skill for their capture than the less intelligent molluscs or crustaceans, and were probably not used at so early a date. They are by no means equally distributed inall seas, and their distribution shows many points of interest.
We must notice, in the first instance, that the waste of the land is of great importance in feeding marine forms, whether directly or indirectly. Marine animals, therefore, occur most abundantly over the Continental Shelf, where they are within reach of the food brought down by the rivers from the land. Again, many fish, or the organisms upon which fish feed, depend largely upon those minute plants called diatoms which float in the upper layers of the waters of the ocean. These are especially abundant in the colder seas, which doubtless helps to explain the abundance of fish in high latitudes. These diatoms, like many other small organisms in the sea, are swept about by the ocean currents, whose course greatly influences the movements of fish.
We saw in the case of forests that hot climates conduce to a great variety of species, while in colder climates the species are few, but the number of individuals very great. Something of the same sort seems to occur with fishes. In warm seas the number of species is very great, while in colder seas there are fewer species, but those which dooccur are sometimes found in vast numbers. Fortunately for man these prolific northern species are often edible, whereas in warm seas poisonous or inedible forms are common. The valuable cod family is found chiefly in high latitudes.
The consequence of the facts just described is that valuable fisheries tend to occur in cool or cold climates rather than hot ones, and because of the dependence of so many forms on the Continental Shelf, they occur in the northern or land hemisphere rather than in the southern or oceanic one.
The most valuable fisheries in the world seem to be those off Newfoundland, where the broad Continental Shelf, forming the so-called “banks,” feeds myriads of cod. The mingling of the waters brought by the cold Labrador currents with those brought by the warm Gulf Stream perhaps influences this marvellous abundance of fish, as does also the waste brought by the icebergs.
Next to the banks of Newfoundland the most valuable fishing ground is the shallow North Sea, which, as we have seen, lies on the surface of the Continental Shelf. Fish are much more abundant here than on the narrower shelf on the western coast of Britain,and the wealth of the North Sea has been an important factor in the development of the countries bordering it.
The warm, salt, relatively deep, and tide-less Mediterranean is not nearly so rich in food fishes as the more northerly seas, a fact reflected in the large importation of dried fish alike from Newfoundland and from the region of the North Sea. But this is an economic and not a zoological statement, for the Mediterranean is in reality richer in fish species than the North Sea, in this respect, as in some others, approaching tropical regions. Among the economically important fish are the tunny, a very large form allied to the mackerel, which is dried, and sardines and anchovies, which are preserved in oil. Otherwise the fish are eaten fresh, and do not enter into general trade.
Fresh-water fish are abundant all over Europe, but with some exceptions they are not greatly prized in those countries where the better-flavoured marine fish can be obtained. Elsewhere, as in Russia, Germany, and parts of France, they become important.
Much more valuable than fresh-water fish in the strict sense are the various kinds of salmon, which come up the rivers to breed,but spend much time also in salt water. In the rivers of Scotland and Scandinavia salmon are still very important, but the fisheries in both cases are insignificant when compared with those of western North America. Salmon are inhabitants of temperate waters, and in North America do not extend further south than the rivers flowing into the north of the Gulf of California. Off the coast of Alaska and British Columbia, especially the former, they are enormously abundant, and being caught in quantities which far exceed the local demand are largely canned for export.
It is interesting to note that in regard to fresh-water fish, as with marine forms, the northern part of the world is especially rich in edible species, as compared alike with the southern hemisphere and with the tropics. The salmon family is confined to the northern hemisphere, and the carp family, though not peculiar, is largely represented in the north. To it belong the whitefish, which form important food fish in many parts of America. Sturgeon, which are important in Russia, occur in the great rivers of eastern Europe, and in parts of Asia, and also on the eastern coast of North America, and off California.
Turning next to the distribution of landanimals within the European area, the first point is to note that for the globe at large zoologists employ zoogeographical divisions based chiefly upon the distribution of the landmammals. The reasons for this are manifold.
In the first place, mammals are of relatively recent origin, and in taking account of their spread over the globe, we may assume that in broad outline the continents, or at least the deep oceans, were much the same when the existing mammals were evolved as at present. This naturally simplifies the problem, for if we divided the globe into regions on the basis of the distribution of reptiles, for example, we should find it necessary to take account of many differences between the world in which the first reptiles arose and the world as it is at present.
Again, the chances of land mammals passing from one region to another, except by the crossing of land surfaces, are small. Thus the occurrence of similar land mammals in two regions now widely separated is almost certain proof of a former land connection between the two regions. The difficulty which most land mammals find in crossing mountain chains, or deserts, or considerable extents of water,makes it easy to define zoogeographical regions separated from one another by the existence of such “barriers to distribution” as they are called. Finally, mammals are highly organised animals of relatively large size, and their distribution is more easily studied than that of insects, for instance.
Without going into the zoogeographical regions in detail, we may note that there is, as already stated, considerable resemblance between the mammals of Europe, Asia, Africa and North America, that is, of the land hemisphere, while South America, which was for long isolated from North America, has a peculiar and relatively primitive fauna, and Australia, whose isolation has lasted longer, has an even more peculiar and a much more primitive fauna.
When we look at the fauna of the great land mass formed by the continents of Europe, Asia, Africa and North America, sometimes called by zoogeographers theArctogæicrealm, we find that North America differs from the eastern land mass as regards its land mammals in several respects. Though long separated from South America it has been connected long enough for some of the southern forms to find their way northwards, so that wefind skunks, raccoons, and other mammals strikingly different from analogous forms found in the Old World. Again, it is relatively so long since there was any free communication between the eastern and western hemispheres that the two faunas have had time to diverge without destroying the fundamental resemblance.
Beginning with the fauna of the Old World, we find that no effective barrier of any sort separates the animals of Europe, even of western Europe, from the animals of temperate Asia, even of eastern Asia. Right across from the British Isles to Japan, through about a hundred and fifty degrees of longitude, there is great general similarity in the land animals. To the south, on the other hand, the Atlas mountains and the African desert cut off the greater part of the continent of Africa, and eastwards the transverse mountain chains, no less than the difference of climate and the cold, barren nature of the uplands of central Asia, cut off the rich fauna of the peninsula of India with Further India, etc., from the habitable regions of temperate Asia, with their scantier fauna.
We are thus left with the conception of a very large and tolerably uniform zoologicalregion, stretching right across Europe and temperate and northern Asia. This is the Palæarctic region of zoogeographers.
The European section of it is somewhat impoverished as compared with the Asiatic section, partly perhaps because of the effects of the ice, and certainly also because for long ages Europe has been densely populated, and the larger wild animals have thus been exterminated. Asia, with its northern forests and its more southerly steppes, has always been a great reservoir of life, which has periodically overflowed into Europe. Some of these overflowing animals, like the black and the brown rats, succeeded in establishing themselves very firmly; others, like the saiga antelope, died out rapidly except in the extreme east of the European area.
It is possible that further investigation will show that not the mammals only, but land animals in general can be grouped according to their habitat like plants, but so far the attempts made in this direction have been tentative only. Generally, we may say that the mammals of Central Europe are of the woodland type, but no detailed classification into steppe and woodland animals exists. It may be useful, therefore, to indicate the chiefkinds of mammals found in the European area, grouped according to affinity, in the absence of a geographical classification.
Mammals, apart from the egg-laying monotremes, and the marsupials of Australia, are divided into nine orders, and of these, one, that including the anteaters, etc., of South America, Africa and India, is entirely unrepresented in Europe. Another, the Cetaceans, or whales, has no land representatives; and the same is true of the aberrant sea-cows, though their ancestors lived on land and occurred in Egypt.
Excluding these orders we are left with six which have European representatives. These are the following:—
Primates, or monkeys and apes.
Insectivores, or insect-eating mammals, such as moles, shrews and hedgehogs.
Chiroptera, or bats.
Ungulates, or hoofed animals, including horses, cattle, sheep, deer, pigs, etc.
Carnivores, or flesh-eaters, including lions, cats, foxes, dogs, etc.
Rodents, or gnawing animals, among which are rats, mice, squirrels, etc.
ThePrimatesare represented by one form only, the Barbary ape, found in Gibraltar.Batsare numerous, but are of less geographical interest than land forms. The remaining four orders are all important. TheUngulatesinclude the largest land mammals, and their size and conspicuous nature have led to the gradual replacement of the wild forms by domesticated ones. Only a very few, such as deer, wild goats (ibex), the wild boar, the wild sheep (moufflon) of Corsica, manage to survive, and that mostly by aid of special protection. The presence of the large wild forms is incompatible with almost any form of agriculture as is often proved disastrously in Africa, hence man’s ruthless warfare upon them.
But if man has destroyed the large ungulates he has found himself unable even to reduce the numbers of theRodents, who gain in many ways by civilisation. The destruction of their rivals, the grass-eating ungulates, increases their natural food-supply. In South America, where there were very few ungulates till the white man brought his flocks and herds, the rodents were very numerous and reached a great size. Again, the operations of agriculture give the rodents enormous artificial sources of food-supply, and the number of man’s domesticated orsemi-domesticated animals makes him wage a bitter war against the small carnivores, the natural enemies of the rodents. Protected from their enemies, abundantly fed by man’s providence, it is no wonder that these small animals have multiplied greatly.
Their multiplication has been assisted by the fact that they inherit from their early days, when the struggle was keen, an enormous fertility. Many of the rodents are steppe animals, and share with steppe organisms in general the power of periodic multiplication in enormous numbers.
The steppe is a region where the rainfall is normally just enough to ensure a free growth of grass at certain seasons. Variations in rainfall, which perhaps occur in great cycles, may at one time produce a luxuriance of growth which increases the food-supply all round, and at another give rise to semi-desert conditions with a resulting enormous death-rate. The steppe organisms, then, must be very fertile because of the risks of their environment, and the Asiatic overflow is possibly determined by successions of years of abundant rainfall, which increase the number of individuals, followed by a series of years of scanty rain, which make itnecessary for the overflow of population to migrate.
Among examples of European rodents we may mention the very destructive rats, mice and voles, which practically feed everywhere at man’s expense; and the hamster, an Asiatic form which reaches as far west as the Rhine, and stores large quantities of corn and other food in an elaborately made burrow. The hamster has the rodent power of rapid multiplication, and is often terribly destructive to cultivated crops. Rabbits are similarly very destructive where special precautions are not taken. Even the porcupine of southern Europe is capable of doing considerable damage. Less serious enemies of man are such forms as the following:—lemmings; marmots, of which there are two forms, an Alpine and an Asiatic, the latter extending like the other steppe animals into the plains of central Europe; beavers; squirrels; dormice; etc. These examples may be sufficient to illustrate the important points in regard to the rodents—their destructiveness, their fertility, and the fact that many were originally inhabitants of steppes and open plains, but tend, as man clears the forest-land for his own purposes, to extend their range to thecleared land, and to appropriate the new and extensive food-supply furnished by man’s industry.
While the ungulates, because of the nature of their food, must almost necessarily be rather large animals, the carnivores occur both in large and small forms. The tendency is for the large forms to be killed out with the progress of civilisation; thus the lion has wholly disappeared from Europe, wolf and bear are almost gone, but a considerable number of smaller forms still remain, such as badger, genet, wolverene, lynx, wild cat, stoat, marten, weasel, etc. The last order to be mentioned, that of theInsectivores, includes small mammals, such as moles, shrews, and hedgehogs, which feed largely on insects, but may be partially vegetarians.
As was to be expected from the climate and from the peculiar flora, the Mediterranean region possesses a richer fauna than central Europe, both as regards mammals and lower forms. Even the European portion shows considerable African influence.
A few words must be said about other land animals apart from mammals. In regard to birds it is noticeable that the habit of migration, and the fact that the greater partof the continent of Europe lies on the direct line between the northern breeding grounds of many species and the southern winter quarters, gives Europe a very rich bird fauna. The British Islands owe to their peculiarly mild climate a rich bird fauna at all seasons, for while the summer climate attracts many forms for nesting purposes, the mild winter brings many migrants flying from the cold of continental Europe.
In regard to birds as well as to other animals, the Mediterranean owes to its warm climate a richer fauna than countries farther north. Some interesting southern forms, such as pelican, flamingo and ibis, reach this region, though not extending into central Europe, except as stragglers.
The climate of Europe is not hot enough anywhere to lead to the presence of a rich reptilian fauna, but there is, again, a marked increase to the south. It is stated that there are only twenty-one species of reptiles in central Europe, while there are fifty-nine in southern Europe, and no less than a hundred and forty in the Mediterranean region taken in the large sense. Poisonous forms are few, and do not, as in hotter countries, constitute a serious menace to man. Very interestingis the presence of the chameleon in southern Spain, as in north Africa.
Perhaps the most important human aspect of the European reptiles is the presence of numbers of insect-eating forms. In the warmer parts of Europe every wall or patch of rock seems alive with lizards in the summer sunshine, and these must play a not inconsiderable part in the keeping down of noxious insects.
Omitting a great number of other groups, we may say something about insects, which are of enormous importance in human life, both directly and indirectly.
It has been shown of late years that many insects are the sole means by which certain very deadly diseases are transmitted from man to man, or from one animal to another. Almost every few months a new announcement of an insect-carried disease is made, but the most important forms are the following:—Mosquitoes and gnats transmit such diseases as malaria, yellow fever, and more horrible diseases still, due to the presence in the blood of small parasitic worms. Tsetse flies carry sleeping sickness, and also transmit the very fatal fly disease of domesticated animals, a fact which has been and is of great importancein the settlement of Africa. In the case of most diseases there seems to be a close connection between one particular species of insect and a particular disease.
Mosquitoes and gnats are very abundant in many parts of Europe, and the forms belonging to the genus Anopheles, which carry the germ of malaria, are widely distributed. In parts of the Mediterranean area their presence is associated with the prevalence of malaria, which has existed there for a prolonged period, and is believed by some to have had an important bearing upon the fates of the ancient civilisations of the Mediterranean basin.
The regions in Europe affected, or seriously affected, by malaria are diminishing yearly. This is now due to conscious efforts, but a similar process has been going on probably for a long period, for many obscure diseases, notably “ague,” seem to have been forms of malaria. Their disappearance seems to be due to drainage, which diminishes the breeding places of the mosquitoes, and also to the progress of agriculture, for ponds which form on rich, well-manured land are apparently unsuited to mosquito larvæ. The subject is of great geographical importance,for the spread of man over the surface of the globe, and the progress of civilisation must have been influenced in all time by the prevalence of fly-borne disease. Such diseases have hitherto been the greatest obstacle in the way of the civilisation of Africa.
In Uganda extensive tracts of fertile wooded land have had to be abandoned on account of the presence there of the tsetse fly, while, prior to this abandonment, there were districts in which every living soul had been destroyed by the deadly sleeping sickness transmitted by this fly. We can hardly suppose that such facts are without a parallel in human history; and man’s distribution over the surface of the globe, and in detail the distribution of his settlements within a country, have doubtless been greatly influenced by the distribution even of such insignificant creatures as the various kinds of flies.
Even apart from their power of transmitting disease, the blood-sucking flies must have influenced man in his choice of localities for settlements, and must have been an important factor in the process of adjustment to his surroundings. The naturalist Brehm gives an appalling picture of the number andblood-thirstiness of the mosquitoes of the Siberian tundra, which render life almost intolerable there for both man and beast in summer. Even within the British Islands the uncultivated and undrained regions are often badly infested with small blood-sucking flies, and their numbers must have been vastly greater in the old days before drainage and intensive cultivation had reduced them. It is quite possible that some of the anomalies in regard to the spread of particular races of men over the surface can be explained by the varying susceptibility of different races to insect attack, and there can be no doubt that the blood-sucking insects must have had some effect in determining the rapidity or slowness with which particular tracts were colonised by man.
Apart from the blood-sucking flies, there are many other interesting points about the insects of Europe, notably the wealth of beautiful and striking forms which occur round the Mediterranean basin. One of these, which extends northwards and westwards to northern France, is the curious Praying Mantis, a predatory insect belonging to the same order as the locust. It is an eastern form, which, like so many others, hastaken advantage of the mild climate of western Europe to extend its range far beyond what we must regard as its natural limits. In France it shows the effect of relatively unfavourable conditions in the fact that it takes some nine to ten months for the eggs to hatch, whereas in hotter countries the process may take place in a few weeks.
In the warmer parts of Europe a very striking feature is the number and large size of the members of the locust and grasshopper families, whose shrill noise is so characteristic a sound in, for example, the pastures of Switzerland in summer-time. Among the locusts there occur, in many parts of Europe, those migratory forms which possess that power of periodic enormous multiplication which we have already noted so frequently among grassland animals. The migratory instinct only seems to develop when the numbers have greatly increased in any given locality, and in Europe generally the climate does not permit this to take place. It does, however, occur in the south-east of the Mediterranean basin, notably in the island of Cyprus, in Syria, and also in Northern Africa, where locusts sometimes reach the dimensions of a plague.