Their multiplication has been assisted by the fact that they inherit from their early days, when the struggle was keen, an enormous fertility. Many of the rodents are steppe animals, and share with steppe organisms in general the power of periodic multiplication in enormous numbers.
The steppe is a region where the rainfall is normally just enough to ensure a free growth of grass at certain seasons. Variations in rainfall, which perhaps occur in great cycles, may at one time produce a luxuriance of growth which increases the food-supply all round, and at another give rise to semi-desert conditions with a resulting enormous death-rate. The steppe organisms, then, must be very fertile because of the risks of their environment, and the Asiatic overflow is possibly determined by successions of years of abundant rainfall, which increase the number of individuals, followed by a series of years of scanty rain, which make it necessary for the overflow of population to migrate.
Among examples of European rodents we may mention the very destructive rats, mice and voles, which practically feed everywhere at man's expense; and the hamster, an Asiatic form which reaches as far west as the Rhine, and stores large quantities of corn and other food in an elaborately made burrow. The hamster has the rodent power of rapid multiplication, and is often terribly destructive to cultivated crops. Rabbits are similarly very destructive where special precautions are not taken. Even the porcupine of southern Europe is capable of doing considerable damage. Less serious enemies of man are such forms as the following:--lemmings; marmots, of which there are two forms, an Alpine and an Asiatic, the latter extending like the other steppe animals into the plains of central Europe; beavers; squirrels; dormice; etc. These examples may be sufficient to illustrate the important points in regard to the rodents--their destructiveness, their fertility, and the fact that many were originally inhabitants of steppes and open plains, but tend, as man clears the forest-land for his own purposes, to extend their range to the cleared land, and to appropriate the new and extensive food-supply furnished by man's industry.