EVOLUTION AND THE PRESENT AGE 257 
toward the establishment of the theory upon which 
that mighty achievement rests, the theory of the cor- 
relation of forces, or rather, perhaps, of the transform- 
ableness of modes of molecular motion, which is to-day 
the fundamental truth upon which the doctrine of evo- 
lution is based. 
I spoke a moment ago of the great historic impor- 
tance of the domestication of oxen and horses. The 
essential feature of the present day is that instead of 
borrowing motor energy from these noble and benefi- 
cent creatures, we manufacture it through deft manipu- 
lation of the forces of inorganic matter. Already the 
time is visibly approaching when the muscular strength 
of horses and oxen will be among the least of their 
uses to man. The number of horseless carriages that 
one meets on the street increases day by day; and elec- 
tric cars, even in their present clumsy stage of devel- 
opment, are doing much to modify the face of things. 
One of the first effects of railways was to centralize 
industries and enable a greater number of people to 
live upon a given area; and hence one of the charac- 
teristic features of the century, by no means confined 
to America, has been the unprecedented increase in 
the size of cities. Now a visible effect of the short- 
distance electric tramway is to aid the diffusion of 
city populations over increasingly large suburban 
areas. The result will doubtless be to enhance alike 
the comfort of the town and the civilization of the 
country. 
Yet another method of creating motor energy is 
through chemical processes, one of the earliest of which 
was the invention of gunpowder four centuries ago; 
but at the close of the eighteenth century a new era set 
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