260 EVOLUTION AND THE PRESENT AGE 
and the moon does not fall simply because its momen- 
tum keeps it as far away as it can get, exactly like a 
pebble whirled at the end of a string. It remained to 
show that the force of the pull varied directly with the 
mass of the bodies, and inversely, with the squares of 
their distances apart; and then it became necessary to 
know that the planetary motions thus produced would 
agree with what Kepler had shown them to be. The 
successful accomplishment of this task remains to-day 
the great typical instance of a perfect scientific discov- 
ery. It is further memorable as the first successful 
leap of the human mind from the earth on which man 
treads into the abysses of celestial space. Be it ob- 
served that what Newton did was to show that through- 
out the world of the solar system certain things go on 
exactly as they do in your own parlour and kitchen. 
Whether it be in the next street or out on the farthest 
planet, it is equally true that unsupported bodies fall 
and that things whirled try to get away. 
I say, then, that Newton’s discovery was a great step 
toward the unification of nature; it was the first deci- 
sive step in the demonstration that the universe is not 
one thing here and another thing there, but is animated 
by a principle of action that yields similar results wher- 
ever you go. Newton expressed his law of gravitation 
in terms that were universal, and there can be no doubt 
that he believed it to hold true of the stellar regions ; 
yet it is only within the present century that the cor- 
rectness of this latter opinion has been proved by direct 
observation. We may now safely affirm that the whole 
stellar universe conforms to the law of gravitation, but 
we can also go much farther than this. The wonderful 
discovery of spectrum analysis by Kirchhoff and Bunsen 
