. [Articles about birds from National geographic magazine]. Birds. 96 The National Geographic Magazine. Photo by Bailey, Biological Survey GRAY r'OX—NEW MEXICO Study of the use of epidemic diseases— nature's own method of destro3'ing sur- plus population. It has long been known that at irregular intervals, when mam- mals, especially rodents, that live in crowded communities increase till they are very numerous, they are suddenly smitten with an epidemic which almost wipes out the species over a considerable area. In the case of such epidemics a certain number of individuals either are immune t
. [Articles about birds from National geographic magazine]. Birds. 96 The National Geographic Magazine. Photo by Bailey, Biological Survey GRAY r'OX—NEW MEXICO Study of the use of epidemic diseases— nature's own method of destro3'ing sur- plus population. It has long been known that at irregular intervals, when mam- mals, especially rodents, that live in crowded communities increase till they are very numerous, they are suddenly smitten with an epidemic which almost wipes out the species over a considerable area. In the case of such epidemics a certain number of individuals either are immune to the disease or recover from it; for while nature is prodigal with the lives of individuals and wastes them with ap- parent recklessness, she cherishes the species and is chary of exposing one to the risk of elimination. After a few years the animal that has paid the price of too great prosperity again multiplies beyond limits, to be again reduced. Efforts are now being made to obtain cultures of the diseases which prevail among the more destructive of our ro- dents, so that they may be employed in other regions where the animals are pests. Since the cultures mav be renewed from time to time, they can be kept indefinitely and be ready for use as required. If they prove as effective as when employed by nature, the problem of a cheap and re- liable method of dealing with destructive rodents will have been solved. FOX FAEIIING Time was when it might almost have been said that America furnished furs for the world, and even now no inconsider- able part of the fur harvest comes from America. Year by year, however, the harvest is diminishing, while the price of furs is steadily advancing, till the finer and rarer kinds are within the reach of only the very wealthy. Foxes of the more valuable kinds, for instance, once so nu- merous in this country, are now compara- tively scarce. Their fur is so valuable and so much sought for that, instead of trying to discover means to compass t
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