The Democratic advocate. [volume] (Westminster, Md.) 1865-1972, 1877-02-17

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One Of the most important considera.
tions in studying the past history of the
earth, as shown by the distribution of ami
mails, is that which leads us to examine,
first, what means animals of every class
have for dispersal, and second, what bar
riers Nature interposes to prevent the
same. II is necessary part Of the great
struggle for existence, which pervades all
life, that the creature shall encounter not
merely active enemies but passive ones
not merely those which directly threaten
its existence, but those which prevent its
self-maintenance by cutting off its access
to the necessary means Of so doing; and
against these last the organism is often
compelled by force of necessity to oppose
itself Animals, even those which breed
most slowly, increase with rapidity out
of all proportion to the available food in
any specified district which they may in
habit; and therefore all are obliged to
struggle against the obstacles which pre-
vent them wandering in search Of fresh
hunting grounds or pastures.
Whether a certain natural phenomenon
is or is not barrier to further dispersion
depends very greatly upon the class Of an
imals inhabiting the region which it limits.
Thus the elephant viii climb the loftiest
peaks and mountains, traverse rivers, and
range the densest forests; the tiger can
endure the widest extremes Of heat and
cold, and can swim moderate distances;
but on the other hand, the monkeys, for
example, must remain within the limits of
forest vegetation, while the antelopes and
zebras cannot exist otherwise than on the
deserts.
Mr. Alfred Wallace, in his "Geographi-
cal Distribution of Animals" " deyotos some
very interesting pages to the above topic,
considering in some detail the various OBJ
stacles to animal emigration. Climate
seems to be potent boundary to the
travels Of mammals, as there are such ami.
mails as the polar bear and walrus, which
cannot live, in state of nature, far be-
yond the polar ocean. But it is believed
that it is not so much the climate itself as
the change of vegetation consequent on
climate which renders it effective as a bar
rier. 1t appears that valleys and rivers
are often insurmountable obstacles, as ami
mails which naturally exist on hills would
be checked by the difference of vegetation
and of insect life, and also by the unheal-
thy atmosphere often found in valleys
An arm Of the sea over twenty miles wide
cannot be traversed by land animals, by
swimming; but on the other hand, long
voyages are often made by mammals that
are involuntary passengers on uprooted
trees and ice foes. Bate and the cetacea
have exceptional means Of dispersal The
latter, however, find themselves opposed
by temperature, as the polar species cannot
cross the equator, nor can those indigenous
to the tropics venture into the cold polar
waters.
1t would seem that no barrier could
limit the range of birds, aud that conse-
quently they must be the most ubiquitous
Of living things; but this is far from being
the case. The petrels and gulls are the
greatest wanderers over the ocean, and
the sandpipers and ployers roam over i'm
mense extents of coasts; but there are
many species which are wholly checked
by natura! obstacles. The ocean presents
an almost absolute barrier to prevent the
birds Of one continent passing over to an
other. Large numbers of birds cannot
exist outside the forest countries; others
cannot soar above the mountain ranges
which bound their inhabited region. Again
the prevalence of their enemies is potent
barrier to birds dwelling in or crossing any
region; and where nest-hunting quadru-
peds, such as monkeys, abound, they are
comparatively scarce.
We now reach that very interesting phe-
nomenon known as migration; and here
must be drawn a distinction between the
true migrations Of fishes and birds and the
periodical movements of certain mammalia.
Thus, in summer, monkeys ascend the
Himalayas to heights of 10000 and r.
600 feet; in dry seasons antelopes move
southwark toward the Cape of Good Hope.
These differ from the great movements of
fishes and birds, since such take place in
large bodies and often to considerable dis
tances. Migration may be looked upon as
an exaggeration of habit, co lon to all
locomotive animals, of moving about in
search of food; and in birds, it is especial
ly exaggerated by their powers of fight
and the necessity of providing soft insect
food for their unaedged young. In North
America, every grade of migration is found,
from that peculiar to species which merely
shift the limits Of their range few hun
dred miles ESQ that in the central parts of
the area the species is permanent resi-
dent, to others which move completely
over 1000 miles of latitude. So that, in
all the intervening districts, such species
are only known as birds of passage. There
are many curious facts peculiar to migra-
tion, notably that of birds returning, year
after year, to build nests in the same spot:
a local attachment which prevents their
wandering into localities unsuitable for
them. Also that the old birds migrate
first, the young following at random. This
indicates the absence of imperative in-
stinct in the habit, and it also accounts
for the diminution in numbers Of the young
that return. On the succeeding year,
however, the young profit by their expe-
rience, and fly when the old birds do.
Another curious fact, however, in favor of
instinct, is that agitation" of Gagged birds
at the time when their wild companions
are migrating. This, however, Mr.. Wal.
lace considers to be due to social excite
ment, due to the anxious cries Of the mi
grating birds, and to be ascribable to some
strong social emotion, gradually developed
in the race by the circumstance that all
who, for want ofsuch emotion, did not Join
their fellows inevitably perished. The long
flights Of, some birds, without apparently
stopping on the way, is thought to be in
explicable, as well as their finding their
nesting place Of the previous year from a
distance Of many hundreds or even thou.
sand miles. But the observant powers Of
animals are very great; and birds flying in
the air may be guided BY, the physica
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