420 
The Evolution of Scientific Knowledge. [July, 
increase in integration and dependence of parts, cannot, I 
think, be for a moment doubted. And the views of Hippar- 
chus, as developed by Ptolemy, may perhaps be looked upon 
as the culmination of the evolution of the geocentric idea, 
while the later history of Astronomy exhibits the evolution 
of the heliocentric idea. 
A scientific theory in some respedts resembles an organism, 
and especially in this — that it must harmonise with its en- 
vironment or die. The environment of theory is fadt. So 
long as a theory is in harmony with the known fadts of 
Nature it can exist, the development of a theory being its 
modification in accordance with newly-discovered fadts. 
But when the plasticity of a theory ceases, — when it refuses 
to accommodate itself to fadt, — its days, are numbered, and 
it must give place to a more fortunate rival in the struggle 
for existence. In this way the Earth-centre theory of our 
system, organised by Hipparchus and developed by Ptolemy, 
had to give place to the Sun-centre theory, foreshadowed by 
Pythagoras and worked out by Copernicus. As Whewell 
truly remarks, so long as the positions only of the heavenly 
bodies were considered, the hypothesis of Hipparchus is a 
close representation of the truth ; but when once the pro- 
cesses of measurement gave sufficiently accurate results 
with respedt to the distances of these bodies, the theory and 
the environing fadts were out of harmony, and the theory 
was doomed. And when Galileo discovered, with the newly- 
invented telescope, in the system of Jupiter and his moons, 
a model of the Solar System ; when he found that Venus, 
in the course of her revolution, assumes the same succession 
of phases which the moon exhibits in the course of a month ; 
and when Kepler observed the transit of Mercury, and 
Horrox the transit of Venus ; then the fate of the old hypo- 
thesis was sealed, and the success of the new theory was 
secured. . ... 
Copernicus, however, retained the conception of circular 
motion, and the consequent existence of epicycles. But the 
idea of epicycles, like the geocentric idea, ere long ceased to 
be in harmony with the environment of fadt. Kepler, we 
are told, attempted to reconcile the theory of Mars to the 
theory of eccentrics and epicycles, the event of which was the 
complete overthrow of that hypothesis, and the proposition 
in its stead of the theory the central truth of which has long 
since been abundantly established, that the planets move in 
ellipses. And this, be it noted, was a substitution of a more 
complex and integrated kind of motion for a combination o£ 
more simple kinds of motion. 
