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 consistent, 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 



