422 The Evolution of Scientific Knowledge . [July, 
the joint mass of the attracting and attracted bodies, and 
inversely as the square of their distance asunder — holds good 
for the sun’s attractive influence on different planets and on 
the same planet in different parts of its orbit ; for the earth’s 
attractive influence on the moon, and on bodies near the 
earth’s surface ; for the mutual attraction of sun, moon, 
earth, planets, and satellites on each other ; and for the 
attractions of the individual particles of which these masses 
are composed. 
The bearing of these well-known faCts on the theory of 
Evolution will now be evident. Not only does the concep- 
tion of Universal Gravitation exhibit a great advance in the 
ideas of the inter-dependence of the members of the solar 
system,- -not only does it show an onward stride in the inte- 
gration of our knowledge, but it displays also a vast increase 
in the orderly complexity of our views of that system by 
introducing definite conceptions of matter and force in addi- 
tion to those of motion and distance. And here we might 
well leave the history of Astronomy, satisfied that it has 
afforded ample illustration of the law under consideration. 
But for the sake of rounding off the argument attention 
maybe drawn to the advance in the traits which characterise 
evolution implied in the nebular hypothesis of Kant and 
Laplace, which affords a not improbable, conception of the 
mode of development of our system, and implied also in the 
results of modern speCtroscopic researches, which teach us 
that the chemistry of the sun and stars is not dissimilar to 
the chemistry of the earth, and which have raised the 
physics of the sun to the rank of an independent science. 
When we take into consideration, too, the conceptions con- 
cerning stellar distribution, started by Wright, developed by 
William Herschel, and now criticised and opposed by ProCtor, 
and add to these the results concerning the motion of the 
sun through space, the velocities of motion of certain stars 
determined by Huggins, and the “ drifting” of star groups 
inferred by ProCtor, we shall not lack instances of advance 
in definiteness of knowledge, in inter-dependence of ideas, 
and in complexity of our total conception of the phenomena 
of the heavens ; while the labours of Schiaporelli, Huggins, 
Donati, Lockyer, and Sir William Thomson, the results of 
which point to an intimate connection between nebulae, 
comets, meteorites, and falling stars, bring into view a pro- 
portionate advance in the integration of that knowledge. 
If now we pass from our knowledge of the solar and 
sidereal system to that of the crust of the earth, we shall 
find many faCts which point clearly in the sanre direction. 
