302 PRINCIPLES OF ANIMAL BIOLOGY 



about the plants, and extending their underground runs from root to root, they 

 either killed or seriously injured the alfalfa. By November they had destroyed 

 so large a percentage of the plants that many fields were plowed up as hopelessly 

 ruined. They attacked also the roots of trees, seriously injuring or quite 

 destroying orchards. They killed most of the young shade trees planted along 

 ditches, and so completely girdled large Lombardy and silver poplars that in 

 some cases they caused the death of even such hardy trees. 



"By January, 1908, in fields where the mice had existed by thousands the pre- 

 vious summer and fall, comparatively few, possibly 200 to 500 to each acre, re- 

 mained. The border of the destroyed district was about 6 mUes below Lovelocks, 

 and the mice were gradually moving farther down the valley. In the area below 

 this mice were somewhat in excess of normal numbers and in several centers of 

 abundance had seriously injured fields. Even where most abundant, along the 

 lower border of the affected area, they did not exceed 1500 to the acre. In the 

 winter they attacked every availab'e food supply. Small willows and even grease- 

 wood bushes about the borders of fields were stripped of all the bark within reach, 

 and horse and cattle droppings were gnawed to pieces for the food they contained. 

 Alfalfa roots, however, were the food supply on which the mice were chiefly 

 dependent." 



Apparently sporadic migration, as these irruptive movements may 

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

 are not in the cases observed able to maintain themselves in the invaded 

 regions. However, it is at least not impossible that at times such irrup- 

 tions 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 spo- 

 radic migration. 



Normal Migration. — In addition to the above types of migration, 

 and some variations of them, every species has a kind of movement which 

 may be called normal migration. This is best seen in free-moving terres- 

 trial forms. Among the individuals of such forms there is constant move- 

 ment; and either as individuals or as flocks they wander about over a 

 larger or smaller area, according to their powers of locomotion, to seek 

 food, escape enemies or find mates. After the young are born they also 

 begin to wander about in search of food, with the additional incentive of 

 finding a new home. The individuals on the outskirts of the range thus 

 constantly tend to invade new territory, and where not delimited by un- 

 favorable conditions the form gradually spreads. In the absence of 

 physical barriers the individuals of a species are held together by breeding 

 instincts, so that each individual does not tend to wander far away from 

 its fellows, or if it does it dies without progeny. It is for this reason that 

 the spreading from the periphery of the range is relatively gradual. 

 When the powers of migration and propagation are great the movement 

 becomes more rapid. An illustration of the rapid spreading of a species 



