304 PRINCIPLES OP ANIMAL BIOLOGY 



Again if a human familj' moved gypsy fashion, only on one day of each 

 week, and not more than three miles, then it would wander about 156 

 miles each year, and the Mongolians, crossing Behring Strait might have, 

 at this rate, reached the Straits of Magellan in about fifty years. It is 

 thus beheved by zoogeographers that normal migration is at once the 

 general and most effective method of extending the range and that it is 

 sufficient to account for the size of the range of most animals. 



Accidental Dispersal. — Reference to accidental dispersal has been 

 made in the discussion of discontinuous ranges. Animals are sometimes 

 carried on rafts or floating logs or are blown by the wind beyond their 

 normal range. Marine birds, such as the gannet, are occasionally, during 

 storms, blown inland from the Atlantic Ocean, as far west as Michigan, 

 and a number of observers in the tropics have noted terrestrial animals 

 on floating logs and rafts in the rivers and even out at sea. It has often 

 been asserted that this method of dispersal, which may be called acciden- 

 tal, is efficacious in extending the range, but the claim has very little 

 supporting evidence. The possibility that islands may have received 

 certain forms by accident is not to be ignored; but there are many diffi- 

 culties in accounting for the entire faunas of islands in this way. Some 

 of these difficulties are the inability of some forms to survive a long sea 

 voyage, the fact that many island forms, such as the giant tortoises, 

 could not possibly be carried on rafts or blown by the winds, the necessity 

 that in the higher animals at least a pair of individuals or a pregnant 

 female be landed if the foi'm is to be perpetuated, etc. But the greatest 

 obstacle to the acceptance of accidental dispersal as an effective method 

 of extending ranges lies in the fact that actually observed cases of acci- 

 dental dissemination beyond the range of a form are very few and mostly 

 open to question. A conservative view is that it may operate at rare 

 intervals and for certain forms, and most often over short distances. 



Distribution by Man. — Human agency is responsible for the intro- 

 duction of animals and plants to new regions in a few cases that are well 

 known, and probably in numerous cases of which we are ignorant. In- 

 troduction ma}'' have been designed, or accidental. Sometimes the 

 forms thus introduced have flourished exceedingly in the new range, 

 witness the rabbit in Australia, the cotton boll weevil in southern United 

 States, and the English sparrow in America. These instances lend color 

 to the view that many species, living with moderate success or with diffi- 

 culty in regions now occupied by them, require only to be transported 

 to other places to multiply with rapidity or even to become dominant 

 forms. It is not improbable that species that appear to be on the road 

 to extinction owe the reduction in their numbers either to changes in 

 their environment or to changes in themselves, and that removal to a 

 habitat not ordinarily accessible to them would spell the difference 

 between destruction and rehabilitation. 



