But as animals rarely have an unoccupied country to breed in, and as the food in any one district is strictly limited, their natural tendency is to roam in every direction in search of fresh pastures, or new hunting grounds. In doing so, however, they meet with many obstacles. Rocks and mountains have to be climbed, rivers or marshes to be crossed, deserts or forests to be traversed; while narrow straits or wider arms of the sea separate islands from the main land or continents from each other. We have now to inquire what facilities the different classes of animals have for overcoming these obstacles, and what kind of barriers are most effectual in checking their progress. _Means of Dispersal of Mammalia._--Many of the largest mammalia are able to roam over whole continents and are hardly {11}stopped by any physical obstacles. The elephant is almost equally at home on plains and mountains, and it even climbs to the highest summit of Adam's Peak in Ceylon, which is so steep and rocky as to be very difficult of ascent for man. It traverses rivers with great ease and forces its way through the densest jungle. There seems therefore to be no limit to its powers of wandering, but the necessity of procuring food and its capacity of enduring changes of climate. The tiger is another animal with great powers of dispersal. It crosses rivers and sometimes even swims over narrow straits of the sea, and it can endure the severe cold of North China and Tartary as well as the heats of the plains of Bengal. The rhinoceros, the lion, and many of the ruminants have equal powers of dispersal; so that wherever there is land and sufficient food, there are no limits to their possible range. Other groups of animals are more limited in their migrations. The apes, lemurs, and many monkeys are so strictly adapted to an arboreal life that they can never roam far beyond the limits of the forest vegetation. The same may be said of the squirrels, the opossums, the arboreal cats, and the sloths, with many other groups of less importance. Deserts or open country are equally essential to the existence of others. The camel, the hare, the zebra, the giraffe and many of the antelopes could not exist in a forest country any more than could the jerboas or the prairie marmots. There are other animals which are confined to mountains, and could not extend their range into lowlands or forests. The goats and the sheep are the most striking group of this kind, inhabiting many of the highest mountains of the globe; of which the European ibex and mouflon are striking examples. Rivers are equally necessary to the existence of others, as the beaver, otter, water-vole and capybara; and to such animals high mountain-ranges or deserts must form an absolutely impassable barrier.