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 accumulating 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 mass 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 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 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; 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 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 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 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 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 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 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 the _Arctogæic_ realm, 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 we find 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 zoological region, 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 chief kinds 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.
The _Primates_ are represented by one form only, the Barbary ape, found
in Gibraltar. _Bats_ are numerous, but are of less geographical
interest than land forms. The remaining four orders are all important.
The _Ungulates_ include 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 the _Rodents_, 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 or
semi-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 it necessary 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 the cleared 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
the _Insectivores_, 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 part of 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 interesting is 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 importance in 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 and blood-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, has
taken 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.
We may add to this account of land animals a few details on the land
mammals of North America. The great point of contrast here is that
Europe, from the beginning of the historic period, has always been a
relatively well-peopled region, while in America, prior to the advent of
the white man, the population was scanty. There was thus far more room
in North America than in Europe for great flocks of large mammals. Thus
the plains and prairies carried great herds of bison, while to the north
there were other herds of reindeer, which were never tamed by the
inhabitants of North America as they were in the Old World by the Lapps
and others. The musk-ox is another interesting animal found in the north
of America. It once also lived in Europe, but died out long ago. Just as
the coniferous forest and tundra in Asia produce many small fur-bearing
animals, so do the forest and tundra of North America. Deer are present
as in the Old World, though they are of different types, and there is a
curious animal known as the prong-buck which is peculiar. Wild sheep
occur as they do in Europe, but no wild horse nor ass roams the plains
of America as they roam to-day the wastes of Asia. Without going into
further detail, we may say generally that as regards wild animals, no
less than as regards wild plants, North America shows a closer
resemblance to Asia than to that favoured peninsula of Asia which the
geographers call Europe.