150 ANIMALS OF LAND AND SEA 



superior. But let me say that of all the fleas the largest known, 

 about a quarter of an inch in length, has so far been found only 

 in the District of Columbia or just across its borders. This 

 flea is quite innocuous, as it lives on field mice. 



With us the humble angle-worms are never very large, but 

 in other places some kinds grow to a great size. The largest, 

 from 4 to 6 feet long, much larger than many of the snakes, 

 are from Tasmania. 



Some sea creatures are immensely larger than any of the 

 land creatures, as will be seen in the succeeding pages. 



While size in the abstract is interesting, the larger animals of 

 whatever group are relatively unimportant. They need abun- 

 dant food, and are therefore relatively few in numbers, while 

 being large very many enemies converge upon them, internal 

 as well as external. A relatively slight decrease from any 

 cause in their food supply endangers their existence; too 

 much activity on the part of internal parasites enfeebles them 

 and has the same effect as a reduction in the food supply. 

 The balance between them and their environment is too deli- 

 cate to admit of any alteration. If their territory is invaded 

 by predaceous creatures able to feed upon them, as a rule they 

 soon become extinct. 



It is the smaher animals that dominate the world and with 

 which we must compete. The elephants and most of the wild 

 hoofed animals would long have been extinct had it not been 

 for numerous protecting laws. For us the rabbits, rats and 

 mice are of far more significance, while the little insect eating 

 birds are more important than the condors or the ostriches. 

 And further down the scale the smaller insects and the minute 

 protozoans are far more important still. 



