1880.] The Evolution of Scientific Knowledge. 421 
As we now know, indeed, the motion substituted is even 
more complicated than Kepler supposed. For not only does 
the ellipse itself revolve slowly round the sun, but its. shape 
undergoes change, being sometimes more nearly circular 
than at others, while, at the same time, the plane of the 
planet's motion oscillates about a mean position, thus it 
comes about that the ellipse which accurately represents the 
planet’s motion at one part of its course does not accurately 
represent that motion at another part of its course. 
But the indefatigable industry of Kepler, besides esta- 
blishing the elliptical theory, led him to the discovery of two 
other fundamental laws of formal astronomy,— that the line 
drawn from sun to planet sweeps over equal areas in equal 
times ; and that the square of the time taken to describe a 
planet’s orbit, divided by the cube of its mean distance from 
the sun, is a fraction which is the same for every planet of 
the system. And these formed the bases for the grand gene- 
ralisation which was inevitably to follow. , 
For the next great step in astronomy was Newtons 
splendid induftion. So gigantic was the onward stride then 
made— a stride without parallel in the history of Science 
that it seems at first sight impossible to reconcile it with the 
gradual advance implied in a development by Evolution. 
S But a closer study of history makes evident the parallel 
but imperfea generalisations which were simultaneous with 
this more perfect and exaa generalisation. While Newton 
at Oxford was pondering on cosmical gravitation, Borelli in 
Florence was publishing his theory of the “ balancing of the 
planets,” arising, as he conjectured, from the equality of an 
“ appetite for uniting themselves with the globe round which 
they revolve,” and the “ tendency to recede from the centre 
of revolution.” While Newton was preparing his Prin- 
cipia,” Huyghens, Wren, Halley, and others seem to have 
possessed a general idea that the attractive force exercised 
by the sun varies inversely as the square of the distance 
from the centre. Hooke even went so far as to claim priority 
in publication to Newton himself. And it is undoubtedly 
true, as Whewell points out, that Hooke s assertion was prior 
to Newton’s demonstration. Francis Bacon, again, had not 
only speculated on the mutual attraction of the particles of 
matter, but devised an experiment to ascertain whether 
the gravity of bodies to the earth arose from an attraction 
of the parts of matter towards each other, or was a tendency 
towards the centre of the earth.” But these were but foie- 
shadowings of the truth. It remained for Newton to demon- 
strate that the same law-that the attraction is directly as 
