An illustration from the opposite extreme of the animal world will show that the breeding propensities of the largest existing animal are rivalled by those of the smallest. “The aphis or plant louse,” says Dr. D. Kerfoot Shute in “A First Book in Organic Evolution,” “is so very prolific that it has been estimated that the tenth brood of one female alone would contain more ponderable matter than all the population of China,—estimating this population at five hundred millions.” The rabbits introduced into Australia, where they found conditions ideally favorable to their increase, have multiplied so exceedingly that they have become a veritable pest. In a brief period, the progeny of a few rabbits has multiplied into millions, and these creatures have become so destructive of agriculture that rabbit-killing parties are organized from time to time and tens of thousands of the animals are killed with sticks and stones, in open places, the object being to exterminate as many rabbits as possible in the interest of the farmers’ crops. In 1850, some enterprising citizens of Brooklyn imported a hundred English sparrows and gave them to the free city air. A little later, New York City imported two hundred and twenty of the noisy birds and liberated them in the parks of the metropolis. Rochester imported one hundred of the birds, and Philadelphia, with generous public spirit, opened the bosom of brotherly love to one thousand of the little strangers whose perpetual chatter formed a striking contrast to the worshipful quiet dear to the heart of the Quaker City. In the parks, among the trees, and in the streets of these cities, the sparrows—at home everywhere—twittered and quarreled in their friendly way, and multiplied. In twenty-five years the children of these birds had spread over five hundred square miles. Ten years later they had flung their domain over fifteen thousand square miles, and men were beginning to doubt the wisdom of those who had invited them to our shores. Since then the ubiquitous sparrow has about completed the conquest of the continent. Darting like an arrow amongst horses’ feet, or dodging hurrying automobiles, his legions are familiar sights among the traffic of busy streets. He builds his rude nest under the eaves of houses, and in the architectural pockets around the roofs of public buildings, and thrives and multiplies through his wary association with man. The phenomenal increase in the number of sparrows in North America during a period of seventy years has been due to the gregarious habit of these birds and to the favorable character of the environment. [Illustration: FIG. 43.—JOE AND SALLIE AT HOME. A Chimpanzee couple of Edward’s Zoological Exhibition. From Dr. Paul Carus’s “The Rise of Man.” Courtesy of The Open Court Publishing Co.]