CHAPTER II. 
THE MEANS OF DISPERSAL AND THE MIGRATIONS OF ANIMALS. 
All animals are capable of multiplying so rapidly, that if a 
single pair were placed in a continent with abundance of food 
and no enemies, they might fully stock it in a very short time. 
Thus, a bird which produces ten pairs of young during its life- 
time (and this is far below the fertility of many birds) will, if 
we take its life at five years, increase to a hundred millions in 
about forty years, a number sufficient to stock a large country. 
Many fishes and insects are capable of multiplying several 
thousandfold each year, so that in a few years they would reach 
billions and trillions. Even large and slow breeding mammals, 
which have only one at a birth but continue to breed from eight 
to ten successive years, may increase from a single pair to ten 
millions in less than forty years. 
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 mam- 
malia are able to roam over whole continents and are hardly 
