The geographical distribution of animals, with a study of the relations ...
The geographical distribution of animals, with a study of the relations ... The geographical distribution of animals, with a study of the relations ...
CHAPTEE II.THE MEANS OF DISPERSAL AND THE MIGRATIONS OF ANIMALS.All animals are capable of multiplying so rapidly, that if asingle pair were placed ina continent with abundance of foodand no enemies, they might fully stock it in a very short time.Thus, a bird which produces ten pairs of young during its lifetime(and this is far below the fertility of many birds) will, ifwe take its life at five years, increase to a hundred millions inabout forty years, a number sufficient to stock a large country.Many fishes and insects are capable of multiplying severalthousandfold each year, so that in a few years they would reachbillions and trillions. Even large and slow breeding mammals,which have only one at a birth but continue to breed from eightto ten successive years, may increase from a single pair to tenmillions in less than forty years.But as animals rarely have an unoccupied country to breedin, and as the food in any one district is strictly limited, theirnatural tendency is to roam in every direction in search of freshpastures, or new hunting grounds.In doing so, however, theymeet with many obstacles. Kocks and mountains have to beclimbed, rivers or marshes to be crossed, deserts or forests to betraversed ;while narrow straits or wider arms of the sea separateislands from the main land or continents from each other.Wehave now to inquire what facilities the different classes ofanimals have for overcoming these obstacles, and what kind o£barriers are most effectual in checking their progress.Means of Dispersal of Mammalia.—Many of the largest mammaliaare able to roam over whole continents and are hardly
chap, ii.] DISPERSAL AND MIGRATION. 11stopped by any physical obstacles. The elephant is almostequally at home on plainsand mountains, and it even climbs tothe highest summit of Adam's Peak in Ceylon, which isand rocky as to be very difficult of ascent for man.so steepIt traversesrivers with great ease and forces its way through the densestjungle. There seems therefore to be no limit to its powers ofwandering, but theof enduring changes of climate.great powers of dispersal.necessity of procuring food and its capacityThe tiger is another animal withIt crosses rivers and sometimes evenswims over narrow straits of the sea, and it can endure thesevere cold of North 'China and Tartary as well as the heats ofthe plains of Bengal. The rhinoceros, the lion, and many of theruminants have equal powers of dispersal ;so that wherever thereis land and sufficient food, there are no limits to their possiblerange.Other groups of animals are more limited in their migrations.The apes, lemurs, and many monkeys are so strictlyadapted to an arboreal lifethat they can never roam far beyondthe limits of the forest vegetation. The same may be said ofthe squirrels, the opossums, the arboreal cats, and the sloths, withmany other groups of less importance. Deserts or open countryare equally essential to the existence of others. The camel, thehare, the zebra, the giraffeexist in a forestprairie marmots.and many of the antelopes could notcountry any more than could the jerboas or theThere are other animals which are confined to mountains, andcould not extend their range into lowlands or forests. The goatsand the sheep are the most striking group of this kind, inhabitingmany of the highest mountains of the globe ;of which theEuropean ibex and mouflon are striking examples. Eivers areequally necessary to the existence of others, as the beaver, otter,water-vole and capybara ; and to such animals high mountainrangesor deserts must form an absolutely impassable barrier.Climate as a Limit to the Range of Mammals.—Climate appearsto limit the range of many animals, though there is some reasonto believe that in many cases it is not the climate itself so muchas the change of vegetation consequent on climate which producesthe effect. The quadrumana appear to be limited by climate,
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chap, ii.] DISPERSAL AND MIGRATION. 11stopped by any physical obstacles. <strong>The</strong> elephant is almostequally at home on plainsand mountains, and it even climbs to<strong>the</strong> highest summit <strong>of</strong> Adam's Peak in Ceylon, which isand rocky as to be very difficult <strong>of</strong> ascent for man.so steepIt traversesrivers <strong>with</strong> great ease and forces its way through <strong>the</strong> densestjungle. <strong>The</strong>re seems <strong>the</strong>refore to be no limit to its powers <strong>of</strong>wandering, but <strong>the</strong><strong>of</strong> enduring changes <strong>of</strong> climate.great powers <strong>of</strong> dispersal.necessity <strong>of</strong> procuring food and its capacity<strong>The</strong> tiger is ano<strong>the</strong>r animal <strong>with</strong>It crosses rivers and sometimes evenswims over narrow straits <strong>of</strong> <strong>the</strong> sea, and it can endure <strong>the</strong>severe cold <strong>of</strong> North 'China and Tartary as well as <strong>the</strong> heats <strong>of</strong><strong>the</strong> plains <strong>of</strong> Bengal. <strong>The</strong> rhinoceros, <strong>the</strong> lion, and many <strong>of</strong> <strong>the</strong>ruminants have equal powers <strong>of</strong> dispersal ;so that wherever <strong>the</strong>reis land and sufficient food, <strong>the</strong>re are no limits to <strong>the</strong>ir possiblerange.O<strong>the</strong>r groups <strong>of</strong> <strong>animals</strong> are more limited in <strong>the</strong>ir migrations.<strong>The</strong> apes, lemurs, and many monkeys are so strictlyadapted to an arboreal lifethat <strong>the</strong>y can never roam far beyond<strong>the</strong> limits <strong>of</strong> <strong>the</strong> forest vegetation. <strong>The</strong> same may be said <strong>of</strong><strong>the</strong> squirrels, <strong>the</strong> opossums, <strong>the</strong> arboreal cats, and <strong>the</strong> sloths, <strong>with</strong>many o<strong>the</strong>r groups <strong>of</strong> less importance. Deserts or open countryare equally essential to <strong>the</strong> existence <strong>of</strong> o<strong>the</strong>rs. <strong>The</strong> camel, <strong>the</strong>hare, <strong>the</strong> zebra, <strong>the</strong> giraffeexist in a forestprairie marmots.and many <strong>of</strong> <strong>the</strong> antelopes could notcountry any more than could <strong>the</strong> jerboas or <strong>the</strong><strong>The</strong>re are o<strong>the</strong>r <strong>animals</strong> which are confined to mountains, andcould not extend <strong>the</strong>ir range into lowlands or forests. <strong>The</strong> goatsand <strong>the</strong> sheep are <strong>the</strong> most striking group <strong>of</strong> this kind, inhabitingmany <strong>of</strong> <strong>the</strong> highest mountains <strong>of</strong> <strong>the</strong> globe ;<strong>of</strong> which <strong>the</strong>European ibex and mouflon are striking examples. Eivers areequally necessary to <strong>the</strong> existence <strong>of</strong> o<strong>the</strong>rs, as <strong>the</strong> beaver, otter,water-vole and capybara ; and to such <strong>animals</strong> high mountainrangesor deserts must form an absolutely impassable barrier.Climate as a Limit to <strong>the</strong> Range <strong>of</strong> Mammals.—Climate appearsto limit <strong>the</strong> range <strong>of</strong> many <strong>animals</strong>, though <strong>the</strong>re is some reasonto believe that in many cases it is not <strong>the</strong> climate itself so muchas <strong>the</strong> change <strong>of</strong> vegetation consequent on climate which produces<strong>the</strong> effect. <strong>The</strong> quadrumana appear to be limited by climate,
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