44 THE SHADOW OF THE WORLD'S FUTURE 
had a limit to its possible population, and that its rate 
of growth diminished in the ratio of its possible future 
increase of numbers to its ultimate population. This 
has again been advanced by Prof. Pearl and Dr Reed 
of the Johns Hopkins University, and they have shown 
that such a law does hold good, with a high approxima- 
sion, for the yeast organism, and for the fly Drosophila 
melanogaster. 
The difficulties of the world’s present rate of de- 
velopment are by no means confined to questions of 
food-supply. A study of the increasing rate at which 
its mineral resources are being used up leads to the 
same conclusion, viz., that we have before us a 
troublous future. For be it noted that the rate of 
exploitation of the earth’s resources of coal, oil, iron, 
copper, zinc, tin, aluminium, and perhaps even of gold, 
etC., is one that has been increasing, and it cannot 
possibly go on increasing as it has in the past. The 
denizens of the earth have greatly raised the standard 
of living for the higher civilisations. This has attained 
to a degree of luxury which must perforce come to an 
end, and that soon.” The known supplies of minerals 
and metals would soon utterly fail, if the accelerating 
rates at which many of thém have been recently 
developed were to be kept up. The exploitation 
of these has been faster even than the population 
has grown. It should be noted, however, that 
such accelerations of the rate at which some of the 
metals have been used, are unlikely to last for more 
than a very limited time, and that the supplies of iron 
and aluminium are practically inexhaustible, while 
those of copper are” very great. The population- 
possibilities of the earth are such, that the correctives 
*0 the present rates of use will come automatically. 
It is not proposed here to give examples from the 
statistics of the Mineral Industry: it must suffice to 
say that the curves showing the rates of advance are