26 ESSENTIALS OF ZOOLOGY 



be balanced, all groups are held in bounds by their enemies. Bal- 

 anced animal communities can be found the world over, and we are 

 only beginning to get a notion of the extensive ramifications of the 

 forces concerned in maintaining that balance. Quite clearly most 

 animals live in a state of repression because relatively few of them 

 become pests and overrun the country. About eighty-five years ago 

 someone who had admired the remarkable spirit of the English 

 sparrow in its native European home thought this hardy little bird 

 would be a cheerful addition on this side of the Atlantic. Conse- 

 quently, a few pairs were landed in Brooklyn. In the short years 

 that have elapsed, this sparrow has proved so hardy and free of 

 enemies here that it is now our dominant bird. 



The story of the rabbit in Australia is likewise an interesting 

 example of the effect of balance or lack of it. Not many years ago 

 Australia had no rabbits. It was hoped and intended by English 

 immigrants there, that a few imported pairs of rabbits would in- 

 crease sufficiently so that the old English sport of riding to the 

 hounds might be developed in Australia. To the surprise and dis- 

 may of these people, the rabbits flourished until now they are jeop- 

 ardizing the enterprises of man. 



Again, we have an example of the effect of the natural agents of 

 repression. The Japanese beetle which was recently introduced in 

 the United States by accident has ravaged the vegetation in several 

 eastern states and threatens other areas. When our investigators 

 went to Japan to study the enemies of the beetle in an effort to find 

 a means of control, they had to search for weeks to find a seriously 

 infested area. So impressed are some biologists becoming with the 

 potential danger of interfering with the natural balance, that even 

 when some irritating pest is under discussion, whose extermination 

 is easily possible, they will advise against it until all phases of the 

 animal's existence are thoroughly investigated. To wipe out this 

 form might remove the check on others that are still more obnoxious. 



Vital Relations of Animals and Plants , 



There are certain single-celled organisms that are claimed as ani- 

 mals by zoologists and as plants by botanists. It is difficult to draw 

 an absolutely clear-cut line of distinction. Of course, it is easy to 

 recognize the extremes. Anyone holding a sunflower in one hand 

 and a frog in the other has no difficulty in determining which is 

 animal and which is plant. The distinctly typical animal forms 



