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Jacques W. (Jacques Wardlaw) Redway.

Elementary physical geography : an outline of physiography online

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A BAKRIKR THAT CERTAIN SPFXIES CANNOT PASS



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water, and many kinds are so light that they are carried
for miles in the air.

The stage of growth and development is a condition of
the greatest danger to the existence of the organism.
During this period it quickly and easily succumbs to the
most trifling changes in its surroundings. At this time,
too, it is apt to be the prey of higher organisms that kill



THE DISPERSAL OF LIFE 305

and devour it. ludeed, so great is the mortality during
the period of development that, in many species, not more
than one or two individuals in many thousand reach the
state of maturity.^

The mature stage of the organism follows that of de-
velopment. In this condition it has but one objective
toward which all its energies tend, namely — the reproduc-
tion of its kind. This accomplished, sooner or later it dies ;
that is, the vital principle leaves it, and it is quickly resolved
into the mineral elements — the " dust " — which gave it ex-
ternal form and structure. Not a few species have special
means for the protection of their bodies, and nearly all
possess special organs for the purj)ose of nutrition, and
the higher species liave organs of locomotion.

Laws of Structure. — Many laws are concerned in the
growth, development, and reproduction of organic forms,
but there are three that govern, directly or indirectly, every
form of life. These are heredity, nutrition, and variation.

The law by virtue of which the germs of organisms de-
velop and mature, each into a form of its own kind, is
called heredity. The germ of a species always reproduces
forms like those of the parents or ancestors. Acorns always
produce oak-trees, animals beget each of its own kind, and
the germ that in the human system produces disease, breeds
nothing but disease of its own kind.'^

A seed or an egg develops into an orgauism that bo-
comes an ancestor of many thousand generations, aggre-
gating millions of individuals. But in obedience to the
law of heredity, the individuals of tlu; last generation will
not very greatly differ from their ancestor, nor will they
differ from one another.

The process by which food, once within the body of an
organism, is decomposed and then nia<l(! a part of the
structure of the organism is called nutrition, or fcoding.



306



PHYSICAL GEOGKAPHY



organism



In obedience to this law, new tissue, that is, flesh, blood,
bones, etc., is constantly being made, and older tissue, no
longer useful, is cast off and destroyed. The number of
substances required in nutrition is few. Nearly three-
quarters of the weight of ever}' organic being consists of
water ; in many instances 97 per cent, is
water. The remaining part is composed
mainly of compounds of carbon, nitrogen,
hydrogen, and phosphorus. The food
must contain all these substances or the
will not mature. As a rule,
plants obtain their food from the min-
eral kingdom, and animals, either
directly or indirectly, from plants.
Variation is the law in obedi-
ence to which organisms are
changed, or change themselves,
to meet the conditions
necessary to their exist-
ence. Thus, under cul-
tivation, the wild rose,
no longer needing its
multitude of stamens,
develops them into pet-
s^ als. Under the condi-
tions imposed by its
environment, the almond
has varied its development by taking the'form of the peach
and the nectarine.

Birds that for long-continued generations have obtained
their food from the water have become either swimmers or
waders, and many species of those that scratch the ground
to obtain food have gradually lost the power of extended
flight. The great diversity observable in the various mem-




VARIATION: ADAPTED TO ITS ENVIRON-
MENT



THE DISPEESAL OF LIFE 307

bers of the dog family is a familiar example of the effects
of variation. The horse of present times has but one toe,
but the ancestors of the species in Miocene times had
three, and in Eocene times four toes on the fore feet.^
The birds of early geological periods were much more rep-
tilian in character than those of present times. Some of
the reptiles, too, have lost their feet and are scarcely a
remove from serpents.

Environment. — Variation of species is the result of
food, temi3erature, and moisture. These are the conditions
with which every organism has to battle for existence,
and these determine all its habits. If the environment of a
species changes, one of three things is pretty certain to
take place : the species dies, it migrates, or else it survives
with changed habits.

Thus, if in a given locality, the raiiif:dl lessens materially,
the turf grass quickly discovers it. In order to obtain the
necessary moisture, an enormous development of rootlets
takes place, and if this development does not procure the
necessary amount of water, the turf grass gradually dis-
appears. If a certain species requires an aggregate of ten
inches of rain, distributed monthly, it will perish if the
rainfall decreases to nine inches, or if there is a drought
of more than thirty consecutive days. It will tlirive and
possibly extend its limits if the annual precipitation in-
creases to twelve inches.

The fruit of the common gooseberry, cultivated in moist
regions, has a smooth surface ; but transplanted to arid
regions and left to grow wild, the berry finally matures,
covered with leatliery spines. Cultivation, which is only
another name for change of environment, has icsultcd in
all the beautiful varieties of roses; it lias juoduced the
domesticated fruits from wild fiuits ; it has made the dif-
ference between the wild fowl and the domestic fowl of



308 PHYSICAL GEOGRAPHY

the same species. Since the territory inhabited by a
species is either enlarged or decreased by a change in
food, temperature, and moisture, and since a change in
any of these factors sooner or later results in variation, it
is evident that the distribution and variation of species is
governed mainly by geographic laws.

Causes that apparently are the most trivial are not in-
frequently attended by far-reaching consequences. For
example, the mongoose was introduced into Jamaica in
order to exterminate the cane-rat, then a menace to the
sugar-planter. The mongoose did not lessen the number
of cane-rats, but it exterminated one or two species of
ground-bird, and wdth their disappearance there came
such swarms of "cattle-ticks" and "grass-lice" that the
existence of cattle-raising was threatened. The ground-
birds had prevented any great increase of the insect
species ; but when the former were killed, the latter be-
came an intolerable pest.

Animals and Plants. — Plants are lower in the scale
of life than animals. A few species excepted, they have
not the power of voluntary motion, and if they possess
the power of sensation at all, the latter is of the very
feeblest degree. They derive their nutrition mainly from
the ground and the air, being able to transform mineral
matter, such as w^ater, lime, potash, carbon, etc., into plant
tissue. With one or two exceptions, plants inhale carbon
dioxide and exhale oxygen.

Plants exhibit only in the feeblest degree, if at all, the
faculty of intelligence, and this is observed only in the
way they seek their food. The roots of a plant will grow
in the direction of water, and the flower will open with
the light and close in the presence of darkness. No spe-
cies is known that will pursue its prey or flee from an
enemy. And the reason is obvious : the plant does not



THE DISPERSAL OF LIFE 309

exist at the expense of other life-forms ; it merely trans-
forms dead mineral matter into living matter, which is to
become the food of higher forms. Nevertheless, the ])laut
contains a vital force that causes it to live, grow, develop,
and reproduce ; and w hen this vital force is spent, the
plant dies.

Animals — even the lowest species — are far more com-
plex in organization than plants. The animal lives bj the
destmction of other forms of life, and therefore, in general,
it must possess the powers of locomotion, prehension, or
grasping, and also some means of defence. All animals
possess intelligence, and some of the higher forms have the
faculty of reason. No exact line of division, however,
can be drawn between animals and plants.

Dispersal of Life. — The distribution of life over the
globe is not a matter of chance ; on the contrary, it exhibits
a character that can result only by the operation of lixed
laws. Moreover it must be examined from two sides,
namely — the means possessed by animals and plants
to disperse and, conversely, the barriers that operate to
prevent dispersal.

The means of dispersal are many. All the higher species
of animals possess the power of voluntary motion. Quad-
rupeds use their feet ; birds fly ; nearly all insects have
at least one stage of development in which they possess
wings ; and fishes swim. Marine currents carry many
species from the place of their birth to distant parts ; and
still other s])eci(!S are carried by floating matter, an<l in
the crop of birds.

Seeds of plants are carried by tln^ winds, by running
waters, and in the crops of birds or in the digestive appa-
ratus of animals. Commerce is responsible for the dis-
persal of most species iised for food .md many tliat are
baneful to humanity.'' In short, almost every organism



310 PHYSICAL GEOGRAPHY

possesses means that, imder orJiuary circumstances would
give it a far wider territory than it uoav possesses.

The natural or unrestricted migi'ation of species presents
an interesting aspect. In the temperate zones, as a rule,
the dispersal has been from west to east ; in the torrid
zone it has generally been in the opposite direction. A
moment's thought will suffice to show the reason for this
law, namely — the direction of atmospheric and marine
currents.

But there are many regions swept by marine currents in
which the species they carry will not thrive, and quite as
many traversed by winds that the winds never sow with
seeds, and the soil never fertilizes. Such extraordinary
effects cannot exist without causes, and these are the
natural barriers to distribution.

Barriers. — The barriers to dispersal are even more
potent than its agents. These may be reduced to two
classes — physiographic harriers and environment. Chief
among the former are the high mountain-ranges, oceans
and deserts.

High mountain-ranges form a tolerably effective barrier
to species not provided with means of locomotion, and the
more extensive the highland the gi-eater the difference of
the species on the opposite sides. There are two reasons
for this. In the first place, if the species are unprovided
with means for migration they cannot cross it ; in the
second place, the conditions of climate on the opposite
sides of high mountains are so different that the species
might not survive, even if transported. The low temper-
ature of the summit of the range might also be fatal.

The ocean and other wide expanses of water are effective
barriers to land plants and animals. A few birds endowed
wdth unusual powers of flight, have crossed the ocean ;
seeds and eggs have also made the passage ; and not a few



THE DISPERSAL OF LIFE



511



species have been transported in vessels. But all these
are accidental migrations, and even then the question of
environment would still remain to be determined.

Deserts present the same difficulties. Few species are
able to cross them ; fewer still to remain in them, and the
barrier once surmounted, there may l)e changed conditions
which still forbid the intrusion of the species.

Environment has been considered in the light of a
cause of variation, but it is far more potent as a barrier to




^



f



"'^^^^^^



^^^^^S^M^^^:^



A DESERT BARRIER



the existence of a species. If a species requires a tempera-
ture not lower than 0° (32° F.), it will perish in a .-limate
having a lower range. If it requires an annual lainfall ..f
thirty inches, it will perish if tlie pr.'cipitation falls to
twenty-nine inches; or if it recpiiresa monthly distributn.n
of rain, it will not survive any considerable nund..r of
droughts of more than thirty days.



312



PHYSICAL GEOGRAPHY



Thus it is seen that every species demands certain con-
ditions of food, temperature, and moisture. If these be of
wide range the species will inhabit a wide geographical
territory- ; if they be narrow in range, the limits of its ex-
istence will be correspondingly narrow. If the proportion,
or degree, or quality change, even minutely, the species
will vary ; if they vary materially the species will perish.




THERE MAY BE ENEMIES THAT OPPOSE THE NEW-COMER



It sometimes occurs, however, that a species, once in-
troduced and acclimated, is unable to maintain itself, or
maintaining itself, is unable to spread to any extent over
a region whose soil and climate are in every way adapta-
ble. There are several reasons for this. The region
may have been already pre-empted by other species that
resist encroachment, or there may be enemies constantly
at work seeking to exterminate the new-comer. As a re-



THE DISPERSAL OF LIEE 313

suit, there are some species capable of general dispersion
that are confined to narrow limits, while others have spread
themselves broadcast over both continents.

Thus, turf -grass is easily cultivated, but it has so many
enemies that in a few localities only does it thrive in a
wild state. The willow, on the contrary, spreads wherever
it is introduced. The ostrich does not extend its territory,
but the rabbit has become a pest in almost every part of
the civilized world.

QUESTIONS AND EXERCISES.- Study the common thistle, the
dandelion, or the winged maple, and show how these species may be
spread.

In the temperate regions of North America in what general direction
will those species depending on the winds for distribution be most
apt to spread ?

Note any instance that has come under your personal observation in
which plants have been carried into new territory by winds, by running
streams, or by waves.

Note any instance within your knowledge in which either a natural
feature or the activity of man has formed a barrier to the dispersion of a
plant or an animal species.

What advantages have each of the following species for dispersal ?
the camel, man, the burdock, the ant, the snake, the cotton plant.

The sting of the tsetse fly, an insect of Africa, is fatal to most cattle,
but the offspring of those that survive, are immune from its attacks ;
how will this fact affect the dispersal of cattle ?

COLLATERAL READING AND REFERENCE

Shalkr.— Nature and Man in North Aiaerioa.
Redway.— The Arid Region of the United States.



NOTES

•Thus, aniont,' plants, these sta^'cs are the se«'(l, tlie si»niutin(4
plant, and the mature, lloweriuf,' stage. In animals tliey are the
egg, the embryo, and the adult individual. Among tli.' 1<'W.t
forms of life the ehange.s are often far iiiorf lompir.x. Most



314 PHYSICAL GEOGRAPHY

insects pass through the forms of egg, larva, pupa, and imago,
and in some species tliere are still other intermediate forms.

" As the enemies to a species increase, its fecundity is apt also
to increase. Thus, the spawn of a female cod-fish aggregates sev-
eral million eggs. If all these were to hatch and mature, the sea
would hold but a few generations.

" Since the discovery of the fact that many diseases are due to
the growth and development of minute organisms within the
human body, the science of surgery and that of sanitation have
been greatly aided. Septicemia, variously known as "hospital
fever " and " blood-poisoning," once the bane of every hospital,
are now comparatively rare, and such diseases as small-pox,
typhoid fever, and cholera may be readily quarantined and
stamped out.

* Because of this struggle, that has been waging ever since life
first appeared on the earth, only the individuals that can best
adapt themselves to circumstances are able to survive. Varia-
tion is not always a gradual change in a whole species ; it is
quite as often a distinctive change in several individuals, and the
transmitted change that marks the descendants.

' The Norwegian rat in America, the Colorado potato-beetle in
Europe, and the English sparrow in the United States are exam-
ples. The Califoi-nia species of the phylloxera, an aphis or
plant-louse infesting the grape-vine, was introduced into France
and almost destroyed the vines of that country. The Russian
thistle at one time threatened to overrun the wheat-fields of the
Mississippi basin, and the strictest means are necessary to keep
it under control. The gypsy moth, whose larvae infests ripening
fruit, has attacked the orchards of the New England States, and an
expenditure of nearly a million dollars a year is necessary to keep
its mischievous woi'k in check.



CHAPTER XATLII

THE GEOGRAPHIC DISTRIBUTION OF PLANTS AND

ANIMALS



Not far from 150,000 species of plant ^ and nearl}' as
many of animal life are known to exist, and new species
are discovered yearly. These are distributed in accordance
with the laws noted in the previous chapter — that is, they
live each in the locality best adapted to it. Plant life
inchides species that vary as widely in form and structure
as the multitude of animal species.

Distribution of Plants. — The distribution ^ of vegeta-
tion may be considered in several aspects, namely — alnm-




FORESTRY OF THE NORTHERN REGION ULU uROWTHb ANU NhW

315



316 PHYSICAL GEOGRAPHY

dance and kind ; and these are best studied Avitli reference
to their regional position or else according to their altitude.
The abundance of vegetation is governed mainly by the
conditions of temperature and moisture. In a climate that
is both warm and moist there is nearly always an abun-
dance of vegetation. Because of this fact, plant life is most
abundant in tropical lowlands, decreasing as the latitude
and the altitude increase. In tropical regions it is profuse ;
in temperate climate, abundant ; in cold regions, scanty.

With reference to the distribution of kind, two factors
have been instrumental — environment and time. In the
earlier geological ages certain species seem to have pre-
vailed at certain centres, and from these they have spread
in various directions. The area over which the species of
a region may have spread is a question chiefly of time ;
the locus, one of environment. The vegetation of a given
region is called its /ora.

With respect to distribution the map on page 318 shows
that five centres exist from which species have spread, or
to which they are characteristic.^ Name them.

The Northern Kegions, Eurasian and American, include
the greater part of the two continents north of the Tropic
of Cancer and that part of Africa north of the Atlas Moun-
tains. The two regions contain, as natives, a large number
of the deciduous trees, giains, and fruits. The grains,
maize excepted, and most of the fruits are characteristic of
the Eurasian ; the redwood, sequoias, sugar-cane, tobacco,
and the yuccas to the American region. The two regions
are separated by the Atlantic Ocean, and though the life-
forms are not identical they are very similar.

The South American region embraces the territory south
of the Tropic of Cancer, both mainland and insular. The
mahogany, cinchona, india-rubber, and rosewood are among
the chief species peculiar to the region.



DISTRIBUTION OF PLANTS AND ANIMALS 317

The African or Ethiopian region includes all of Africa
south of the Atlas Mountains and tropical Arabia. The
baobab, oil-palm, euphorbias, bigonias, the coffee-tree,
several heaths, and the geranium, are among the native
plants peculiar to this region.

The Oriental Region includes the territor}^ south of
the Himalaya Mountains, and most of Malaysia. Among
the principal characteristic species are the spices, the
ebony, sandal-wood, and the melons.

The Australian Region comprises the continent of Aus-
tralia and most of the islands east and north. The tlora
of this area is highly peculiar. The prevailing color of the
vegetation is bluish-green and the leaves turn their edges
to the sun. The eucalyptus or gum trees, the various tree-
ferns, and the jarrah are peculiar to this region. In the
north and east the Australian and Oriental regions over-
lap, and are marked by species characteristic to both. The
Eucalyptus and the tree-fern have been introduced into
California ; the jarrah is much used in the manufacture of
street paving-blocks.

The vertical distribution of species is determined by
altitude Thus at the base of the Himalayas and the
Andes, the flora is tropical ; higher up, the characteristic
species of the temperate zones replace tropical ]ilants ; and
at an altitude of twelve thousand feet, more or less, the
vegetation is distinctly that of polar tyjjes.

Economic Plants. — Most forms of i)lant life have an
important relation to mankind, and this is especially true
of those used as food, as medicine, or in the arts, Ch'uA
among them are the grains and other grasses, tuberous
plants, fruits, those yielding textiles, and those used for
building timber.

The grasses prol)ably extend over a ^vi(l('^ arc.-i th.in any
other family. Of these the sugar-cane and niaiz(^, or Indian



DISTRIBUTION OF i LANTS AND ANIMALS 319

corn, are mitive to the American continent. All the others
belong to the Old World, but have followed the march of
mankind. The members of this famil}- are the sole food
of many hundred species of animals,, and the seeds are
consumed b}^ ever}' race and almost every tribe of man-
kind. The starch they contain gives them their chief
value as a food-stuft'.

Kice is confined chiefly to the marine marshes and
swamp lands of tropical and sub-tropical regions, but there
are one or more species of upland rice. Rice is the staple
food of about one-half the people of the world. It is a
notable fact, however, that in certain parts of China and
India, wheat, little by little, is supplanting it. Pound ft)r
pound its nutrient value is not equal to that of wheat.
Maize, or Indian corn, a native of the New World, is an im-
])ortant food-stuff in temperate and sub-tropical regions.
It is the chief bread-stuff of the " mixed "and native races
of the New AVorld. In the United States and Canada it
is used mainly as animal food, being converted into pork.
Its use, both in tlie form of grain and meat, is largely in-
creasing among the peoples of the Old AVorld. It is also
used in the nianufacture of liquor.

Wheat is the l)read-stuft' of the civilized peoples of the
temperate zones and, is the fuel of the activity and
energy of the world. It is grown in tlie great ])laiiis of
the temperate zones, but it thrives in sub-tropical and
sub-polar regions. How do the t()i)Ographic features of a
plain affect the harvesting and transpoi-tation of wlnat?
How do they affect the evolution of harvesting inachiiicry ?

The world requires about 2,200,000,000 bushels of wh.-at
each year, and the amount required is steadily iiicn;isiiig.
Why? Tln' annual cro]) is somewhat greater ; in ISDS it
was 2,700,000,000 buslu'ls. It is estimated that the maxi-
mum crop pos.sible is not fur from twice this amount.



320 PHYSICAL GEOGRAPHY

About one-fourth of the world's crop is produced iu the
United States.

Kye takes the place of Avheat in many countries, and is
one of the most important crops of liussia and Germany.
A species of oat is native to the North American region,
but the cultivated plant is an imported variety. It is a fa-
vorite food for horses. Barley, about the hardiest of
the grains, is also much favored as a food for horses, but
is employed mainly in the manufacture of malt liquors.
Buckwheat ^ is not a wheat at all, but the nut or fruit con-
tains a large percentage of starch ; hence it is much used
as a food-stuff.

The canes include one of the chief sugar-producing
plants. ^ They thrive best in tropical countries, and are
extensively cultivated in the sub-tropical belts. In oriental
countries the bamboo, a species of cane, is much used as
a buildiner material and in the arts.

The palms, next the grasses, probably yield the greatest
variety of useful products. Cocoa-nuts, dates, sago, sugar,
wine, and oil, are all derived from this family. So far as
moisture is concerned, the palms have a wide range, but
in respect to temperature they are restricted to warm
regions. They occur in both hemispheres.


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Online LibraryJacques W. (Jacques Wardlaw) RedwayElementary physical geography : an outline of physiography → online text (page 21 of 25)