THE DISTRIBUTION OF ANIMALS 437 



Laws of distribution. The laws governing the distri- 

 bution of animals over the earth's surface have been 

 recently* expressed in a simple statement as follows: 

 Every species of animal is found in every part of the earth 

 unless (a) its individuals have been unable to reach this 

 region on account of barriers of some sort; or (^) having 

 reached it, the species is unable to maintain itself, through 

 lack of capacity for adaptation, through severity of com- 

 petition with other forms, or through destructive conditions 

 of environment; or (c) having entered and maintained 

 itself it has become so altered in the process of adaptation 

 as to become a species distinct from the original type. 



Modes of migration and distribution. That animals 

 should be continually trying to extend their range is 

 obvious from what we know of their rapid increase by 

 multiplication. In a region which can provide food for 

 but one thousand wolves, there is a production each year 

 of several times one thousand. These new wolves must 

 struggle among themselves for food, or migrate, if possi- 

 ble, to new regions as yet not inhabited by wolves. The 

 wolfs mode of migration or distribution is walking or 

 running, and so with other mammals except the bats and 

 aquatic forms. Birds and bats can fly, and can thus 

 migrate more swiftly, farther, and over barriers which 

 would stop mammals. Most insects can fly. Worms 

 can only crawl and very slowly at that. Fishes can swim, 

 but if they are in a landlocked sheet of water, they cannot 

 go beyond its confines. Marine animals can migrate 

 from ocean to ocean, and land animals from continent to 

 continent unless checked by barriers (see next para- 

 graph). 



But besides such voluntary and independent modes of 

 distribution long journeyings may be made involuntarily, 

 or by a passive migration as it may be called. Parasites, 



* Jordan and Kellogg's " Animal Life," 1900, p. 274. 



