THE NORTH AMERICAN FAUNA 173 



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 artifically introduced. 



It can be asserted with certainty that there is a general tendency 

 for aquatic animals to disappear. 



Kelation to Man 



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 disap- 

 pear 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 possibil- 

 ity 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 emi- 

 nently 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 rattle- 

 snake 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. 



Fecundity 



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 role in the pro- 

 tection 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 her- 

 self hatched. 



A single pair of flies, warmed to activity in April, have within them- 

 selves the potentiality of producing before October (if every egg laid 



