Modern Geography Part 5

This, no doubt, preserves them against loss of water, but also probably protects against grazing 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 appet.i.te 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 gra.s.s 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 a.s.sociated with the Mediterranean are oleander, the n.o.ble 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 forests of the low grounds are here deciduous. In summer clothed in magnificent foliage, well adapted to give off enormous quant.i.ties 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 p.r.o.nounced in spring before the 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 lat.i.tudes, 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 alt.i.tudes and in high lat.i.tudes 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 condition of 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 gra.s.ses 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 gra.s.sland, 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 pa.s.ses into a region of deserts.

The conditions favourable to the growth of gra.s.s 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 gra.s.ses 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 gra.s.ses little, owing to the fact that they have had time to make their growth.

The abundant natural growth of gra.s.s 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 pa.s.sing 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 gra.s.s 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 pa.s.sing into desert, and to Asia by steppe land pa.s.sing 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 on a 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 h.o.m.ologous 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 western seaboard to 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 great _Sequoia_ (_Wellingtonia_) _gigantea_ which has most impressed popular 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 from the 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 lat.i.tude 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 a.n.a.logous to the steppes of Asia.

Eastward the rainfall increases, and we pa.s.s from the area of unreclaimed pasture to the prairies, now largely laid down to wheat and other food plants. Southward the Great Plains pa.s.s 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 wheat into 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 sh.o.r.es of the Gulf of St.

Lawrence to the extremity of the peninsula of Florida, that is, through about 25 degrees of lat.i.tude, 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 on p. 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 pa.s.s 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 stretching from 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.

CHAPTER VI

THE DISTRIBUTION OF ANIMAL LIFE

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 acc.u.mulating on all sides, but the subject is still rather at the level of collecting information than 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 the _Challenger_ expedition. The results of that expedition appeared in many large volumes, which form a conspicuous feature in any complete scientific library and contain a ma.s.s of useful material. The _Challenger_ expedition 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 sh.o.r.e 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 pa.s.sively 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 land influences, 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; sh.e.l.l-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 sh.o.r.es 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 sh.e.l.l-mounds found on many sh.o.r.es, _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 sh.o.r.e 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 in all 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 lat.i.tudes. 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 do occur 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 lat.i.tudes.

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 quant.i.ties 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 land animals 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 land _mammals_. 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 a.s.sume 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 pa.s.sing 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 ma.s.s formed by the continents of Europe, Asia, Africa and North America, sometimes called by zoogeographers the _Arctogaeic_ realm, we find that North America differs from the eastern land ma.s.s 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 we find skunks, racc.o.o.ns, and other mammals strikingly different from a.n.a.logous 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 j.a.pan, 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 zoological region, stretching right across Europe and temperate and northern Asia. This is the Palaearctic region of zoogeographers.