320 PRINCIPLES OF ANIMAL BIOLOGY 



element of control was removed, and the mice were produced in countless 

 thousands. On some ranches there were as many as 12,000 per acre. 

 Crops were destroyed, trees killed by injury to their roots, and banks 

 of drainage ditches were riddled with their burrows. Great armies of 

 mice moved on to new fields 5 miles or more from the point of first con- 

 centration. Then their hordes disappeared even more quickly than they 

 arose. In the course of three months they dropped to only 200 to 500 

 per acre. No satisfactory explanation of either their increase or their 

 disappearance was ever discovered. 



Apparently sporadic migration, as these irruptive movements may 

 be termed, does not usually result in an extension of range, for the species 

 in the cases observed have not been able to maintain themselves in the 

 invaded regions. However, it is possible that at times such irruptions 

 have brought species into regions where conditions were favorable and 

 thus enlarged the inhabited area. Instances of widely discontinuous 

 range have sometimes been explained, whether correctly or not, by appeal 

 to sporadic migration. 



Accidental Dispersal. — Discontinuous ranges have been more often 

 attributed to accidental dispersal than to sporadic migration. 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 occa- 

 sionally 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 is effica- 

 cious in extending the range. Islands may have received certain forms 

 by accident, but there are many difficulties in accounting for the entire 

 faunas of islands in this way. Some of these difficulties are (1) the 

 inability of some forms to survive a long sea voyage, (2) the fact that 

 many island forms, such as the giant tortoises, could not possibly be 

 carried on rafts or blown by the winds, (3) the necessity that in the 

 higher animals at least a pair of individuals or a pregnant female be 

 landed if the form is to be perpetuated, etc. But the greatest obstacle 

 to the acceptance of accidental dispersal as an eft'ective method of 

 extending ranges lies in the fact that actually observed cases of accidental 

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

 to question. Possibly it may operate at rare intervals, for certain forms 

 and over short distances. 



Man himself is responsible for the introduction of animals and plants 

 to new regions in a few instances that are well known. Sometimes it 

 was done by design, more often by accident as in the transport of rats in 

 ships. The animals carried by man have sometimes succeeded much 

 better in their new locations than in the original ones, witness the rabbit 



