The Theory of Natural Selection. 279 
account for the facts. Once admit the irrational 
principle that we may assume the operation of higher 
causes where the operation of lower ones is sufficient 
to explain the observed phenomena, and all our science 
and all our philosophy are scattered to the winds. 
For the law of logic which Sir William Hamilton 
called the law of parsimony—or the law which forbids 
us to assume the operation of higher causes when lower 
ones are found sufficient to explain the observed 
effects—this law constitutes the only barrier between 
science and superstition. It is always possible to give 
a hypothetical explanation of any phenomenon what- 
soever, by referring it immediately to the intelligence 
of some supernatural agent ; so that the only difference 
between the logic of science and the logic of superstition 
consists in science recognising a validity in the law of 
parsimony which superstition disregards. Therefore 
one can have no hesitation in saying that this way 
of looking at the evidence in favour of natural selection 
is not a scientific or a reasonable way of looking at it, 
but a purely superstitious way. Let us take, as an 
illustration, a perfectly parallel case. When Kepler 
was unable to explain by any known causes the paths 
described by the planets, he resorted to a supernatural 
explanation, and supposed that every planet was guided 
in its movements by some presiding angel. But when 
Newton supplied a beautifully simple physical ex- 
planation, all persons with a scientific habit of mind 
at once abandoned the metaphysical one. Now, to 
be consistcnt, the above-mentioned Professors, and all 
who think with them, ought still to adhere to Kepler’s 
hypothesis in preference to Newton's explanation ; 
for, excepting the law of parsimony, there is certainly 
