=Modes of migration and distribution.=--That animals should be continually trying to extend their range is obvious from what we know of their rapid increase by multiplication. In a region which can provide food for but one thousand wolves, there is a production each year of several times one thousand. These new wolves must struggle among themselves for food, or migrate, if possible, to new regions as yet not inhabited by wolves. The wolf's mode of migration or distribution is walking or running, and so with other mammals except the bats and aquatic forms. Birds and bats can fly, and can thus migrate more swiftly, farther, and over barriers which would stop mammals. Most insects can fly. Worms can only crawl and very slowly at that. Fishes can swim, but if they are in a landlocked sheet of water, they cannot go beyond its confines. Marine animals can migrate from ocean to ocean, and land animals from continent to continent unless checked by barriers (see next paragraph). But besides such voluntary and independent modes of distribution long journeyings may be made involuntarily, or by a passive migration as it may be called. Parasites, for example, are carried by their hosts in all their travels; the tiny Tardigrada and Rotifera, which can be desiccated and yet restored to active life by coming again into water, are carried in the dried mud on the feet of birds or other animals. On floating objects in rivers or in ocean currents land-animals may be carried long distances. Man, the greatest traveller of all, is responsible for the widened distribution of many animals. Thus have come to us in ships from Europe the black and brown rats, the English sparrow, the Hessian fly, the commonest cockroaches of our houses and many other forms. And these animals have been carried involuntarily all over the United States in railway-cars and wagons.