214 SUMMARY 



353. The method of inquiry on which Darwin principally relied, 

 and which enabled him to affect so great a revolution in modern 

 thought, was to compare varieties and races living under one set 

 of conditions with those living under another. Having noted the 



inductions, and 'to make these, by any combinations, however complicated, suffice 

 for proving even such truths, relating to complex cases, as could be proved, if 

 we chose, by inductions from specific experience. Every branch of natural 

 philosophy was originally experimental ; each generalization rested on a special 

 induction, and was derived from its own distinct set of observations and experi- 

 ments. From being sciences of pure experiment, as the phrase is, or, to speak 

 more correctly, sciences in which the reasonings mostly consist of no more than 

 one step, and are expressed by single syllogisms, all these sciences have become 

 to some extent, and some of them in nearly the whole of their extent, sciences of 

 pure reasoning ; whereby multitudes of truths, already known by induction from 

 as many different sets of experiments, have come to be exhibited as deductions 

 or corollaries from inductive propositions of a simpler and more universal 

 character. Thus mechanics, hydrostatics, optics, acoustics, thermology, have 

 successively been rendered mathematical ; and astronomy was brought by Newton 

 within the laws of general mechanics. Why it is that the substitution of this 

 circuitous mode of proceeding for a process apparently much easier and more 

 natural, is held, and justly, to be the greatest triumph of the investigation of 

 nature, we are not, in this stage of our inquiry, prepared to examine. But it is 

 necessary to remark, that although, by this progressive transformation, all sciences 

 tend to become more and more Deductive, they are not, therefore, the less Induc- 

 tive ; every step in the Deduction is still an Induction. The opposition is not 

 between the terms Deductive and Inductive, but between Deductive and Experi- 

 mental. A science is experimental in proportion as every new case, which presents 

 any peculiar features, stands in need of a new set of observations and experiments 

 a fresh induction. It is deductive in proportion as it can draw conclusions, 

 respecting cases of a new kind, by processes which bring those cases under old 

 inductions "(J. S. Mill, Logic, Bk. II. chap. iv. 5). " The discoveries which change 

 the method of a science from experimental to deductive mostly consist in 

 establishing, either by deduction or by direct experiment, that the varieties of 

 a particular phenomenon uniformly accompany the varieties of some other 

 phenomenon better known. Thus the science of sound, which previously stood 

 in the lowest rank of merely experimental science, became deductive when it 

 was proved by experiment that every variety of sound was consequent on, and 

 therefore a mark of, a distinct and definable variety of oscillatory motion among 

 the particles of the transmitting medium. When this was ascertained it followed 

 that every relation of succession or co- existence which obtained between 

 phenomena of the more known class, obtained also between the phenomena which 

 correspond to them in the other class. Every sound being the mark of a particular 

 oscillatory motion, became a mark of everything which, by the laws of dynamics, 

 was known to be inferable from that motion ; and everything which by those 

 same laws was a mark of any oscillatory motion among the particles of an elastic 

 medium, became a mark of the corresponding sound. And thus many truths, 

 not before suspected, concerning sound, become deducible from the known laws 

 of the propagation of motion through an elastic medium ; while facts already 

 empirically known respecting sound become an indication of corresponding 

 properties of vibrating bodies, previously undiscovered " (op. cit., II. iv. 7). " We 

 have thus already come within sight of a conclusion which the progress of the 

 inquiry will, I think, bring before us with the clearest evidence, namely, that in the 



