Wild horses and bison have also vanished before the advances of civilization and the alteration of their homes.  Sometimes the extermination of one pest has resulted in an increase in the number of another through human interference with nature’s equilibrium.  In some of our Western states, a bounty was offered for the scalps of wolves, so as to lessen the number of these predatory foes of sheep.  But when the wolves were diminished in number, their wild food-animals, the prairie dogs, found their lot much bettered, and they have multiplied so rapidly that in some places they have become even more destructive than the wolves.

One of the most remarkable illustrations is that of the rabbits introduced into Australia.  This island continent was cut off from the surrounding lands long before the higher mammals evolved in far distant regions, so that the balance of nature was worked out without reference to animals like the rabbit.  When the first of these were introduced they found a territory without natural enemies where everything was favorable.  They promptly multiplied so rapidly that within a few years their descendants were numerous enough to eat up practically every green thing they could reach.  Two decades ago, the single province of Queensland was forced to expend $85,000,000 in a vain effort to put down the rabbit plague.  The remarkable statement has been made that in some places nature has taken a hand in causing a new type of rabbit to evolve.  Finding the situation desperate, some of the animals have begun to develop into tree-climbing creatures.  The animals exist in such numbers that the available food upon the ground is insufficient for all, and so some elimination results.  But the young rabbits with longer claws, varying in this way on account of congenital factors, have an advantage over their fellows because they can climb some of the trees and so obtain food inaccessible to the others.  If the facts are correctly reported, and if the process of selection on the basis of longer claws and the climbing habit is continued, the original type of animal is splitting up into a form that will remain the same and live upon the ground, and another that will be to all intents and purposes a counterpart of our familiar squirrel.  All the evidence goes to show that squirrels have evolved from terrestrial rodents; if the data relating to Australian rabbits are correct, nature is again producing a squirrel-like animal by evolution in a region where the former natural situation has been interfered with by man.

The laws of biological inheritance have received close and deep study by numerous investigators of Darwinian and post-Darwinian times, because from the first it was clearly recognized that a complete description of nature’s method of accomplishing evolution must show how species maintain the same general characteristics from generation to generation, and also how new qualities may be fixed in heredity as species transform in the course of time.  Before our modern era in biology, the fact of inheritance was accepted as self-sufficient; now much is known that supplements and extends the incomplete account given by natural selection of the way evolution takes place.