68 Department of Conservation of Louisiana 



"To assume that the movement of one mouse in a given 

 direction acts as a stimulus for another one near-by to do 

 the same, or that this movement at least discourages the 

 latter from going in a different or exactly opposite direc- 

 tion, explains the movement once it has begun," Mr. Hall 

 tells us. "But, of course, it could not explain the equal move- 

 ment in all directions from one central point; in fact, if 

 such were the determining factor at the beginning of the 

 migration, a radial migration of the type just described 

 would be prevented. 



"Perhaps, then, a mechanical cause starts the migra- 

 tion ; after the animals have once gone, for a given length of 

 time, in a certain direction, some sort of impulse is de- 

 veloped to continue. But what the stimulus for guidance 

 may be is not clear." 



It must be remembered in the case of the outbreak of 

 house mice in this dry bed of a one-time lake, given over 

 to agricultural pursuits, that the animal causing the plague 

 was an exotic species, an immigrant from the Old World 

 that had not been in that section of California more than 

 fifty years, and that the same relative outbreak in numbers 

 was not observed in the native rats and mice of that section. 

 Mr. Hall found that the conditions of this restricted region 

 were most suitable for the increase of the house mouse, 

 particularly in the abundance of grains, for which they had 

 a great predilection, while the native species of rodents de- 

 sired more green foods. 



Mr. Hall's outstanding observance, was the fact that 

 the farmer-folk of the affected area had kept up an un- 

 ceasing warfare on the birds and beasts of prey — the hawks 

 and owls of the air and the coyotes, foxes, bobcats, skunks 

 and other carnivorous mammals of the land. Although it 

 is known that several factors besides the natural enemies 

 of rodents, native and exotic, determine their numbers, such 

 as favorable climate and abundant plant growth, the fact 

 that mice are small and easy of prey render them the staple 

 food of every one of the carnivorous mammals, while hawks 

 and owls of many species need the tiny rodents for their 

 subsistence. 



