Page:Popular Science Monthly Volume 83.djvu/177

Rh outlets will preclude any general immigration to their waters; and their fauna, for the most part, will be restricted to minute animals, insects and food fishes artificallyartificially [sic] introduced.

It can be asserted with certainty that there is a general tendency for aquatic animals to disappear.

At first thought we might assume that useful species will survive and injurious ones will be wiped out, since man is the all-powerful lord of creation. But the most useful animals are the ones that disappear first. This is because of the unfortunate fact that man is a selfish being and thinks more of the satisfaction of his immediate desires than of the good of his race. Fortunately for the animals concerned, we are waking up to their value and many useful species are now reared in small numbers in a state of semi-domestication and there is a possibility that deer, foxes and many other animals valuable for food or fur will some day be fully domesticated.

On the other hand, it is true that injurious habits tend to bring about the extermination of a species. The venomous snakes are eminently fitted for protection from natural enemies. Their deadly nature has caused man to war upon them and in some localities his warfare has met with so much success that the once dreaded copperhead and rattlesnake are now extinct. The fear in which the pioneers held panthers, wolves, lynxes and other beasts of prey, played a large part in their early extermination.

There is another group of noxious animals against which man rages in impotent wrath. These are the mice and rats, the potato beetles, scale insects, flies and various other injurious insects. Among all of these creatures small size plays an extremely important rôle in the protection of the species. If a mouse weighed 100 pounds instead of less than an ounce, it would be more easily found and killed. The yet smaller size of insects makes them even more difficult to cope with.

Of much greater importance than their small size is the fecundity of these pests. A female deer produces no offspring until three years old and then only one or two a year. The other large animals produce young at about the same rate. But a female rat begins to bear young when six or eight months old and may produce 50 or even more in a single year. A house-fly, under the most favorable conditions, may lay eggs within two weeks of the time the egg was laid from which she herself hatched.

A single pair of flies, warmed to activity in April, have within themselves the potentiality of producing before October (if every egg laid