The Fur Animals of Louisiana 67 



The California investigator found no mice less than 

 three-fourths grown. "Since it is commonly known, from 

 laboratory experiments, that shortage of food curtails or 

 stops reproduction in mice," said Mr. Hall, "the exhaustion 

 of food in the Buena Vista Lake basin is suggested as the 

 probable cause for the apparent absence of young mice 

 among the thousands seen. No evidence of disease of any 

 kind that might be expected to destroy the mice was noted. 

 Judging from what has happened during marked increases 

 in the population of other rodents, some epidemic eventually 

 should, of course, be expected to occur here. 



"Among animals that are subject to such enormous and 

 sudden increases in population, and equally sudden de- 

 creases, it may be inferred that natural selection would 

 operate at an accelerated rate. For instance, with the pres- 

 ent scarcity of food in the Buena Vista Lake region, it can 

 be understood how certain individual mice that are able 

 to survive longest without sufficient food would live to 

 produce offspring, whereas the millions less fit in this re- 

 spect would die and leave no progeny. Assuming that this 

 physiological fitness were heritable, a marked change might 

 occur in this respect within a short series of generations. 

 Perhaps the example chosen is not the most happy one ; but 

 it suggests one, at least partial, explanation of the appar- 

 ently unequal rates of evolution of different descent-lines of 

 animals that come to the attention of the palaeontologist. 

 Certain characters would seem more rapidly to be selected 

 for in a kind of animal that is subject to rapid increases 

 and decreases in population than in a kind whose population 

 remains relatively constant through long series of genera- 

 tions." 



In this California plague of mice, Mr. Hall found that 

 a radial migration resulted, at first, in apparently equal 

 numbers of the mice moving outward from the basin in all 

 directions, and, seemingly, no choice of destination was 

 made by the migrants, there being, according to this author- 

 ity, no stimulus or positive attraction causing all of, or even 

 the majority of, the mice to move in one given direction 

 rather than in another. 



