SECTION 23. GENERALISATION. 349 



without any inner world at all if, following this Conclusion, he 

 consistently excluded all experience, reflection, and classifi- 

 cation. 



Furthermore, we alter the degree of an alleged cause for 

 the purpose of perceiving whether there is a uniformly cor- 

 responding change in the degree of its alleged effects, and 

 vice versa. (This is Bacon's and Mill's method of Concomitant 

 Variation.) We also search for pure instances, for extremes, 

 for that which may be regarded as the mean or the normal, 

 for deviating, morbid, eccentric, border, and transitional in- 

 stances, and for any other remarkable stages or divisions be- 

 tween the extremes or between maximum and minimum, en- 

 deavouring to reduce all exceptions to rules. We inquire whether 

 we are dealing with partly or wholly continuous or qualitatively 

 different states. We gradually eliminate constituents in order 

 to observe the residual phenomena which often prove to be of 

 far-reaching significance both theoretically and practically, and 

 also by stages add others with the same object, as in chemistry. 

 We endeavour to arrange all knowledge in a determinate 

 order atomic weight of chemical elements, specific gravity of 

 substances, influence of time on physical, vital, and cultural 

 components and processes, the evolution of the human eye and 

 other parts of the human body from the earliest manifestation 

 of life on the globe, etc., allowing for veiled or hidden resem- 

 blances and dissimilarities, as in subcutaneous processes or 

 structures in the living or superficially different but really 

 homologous vital functions and parts, as illustrated by the 

 "wings" of the bat, the "fins" of the whale, and the electric 

 organs of fishes. 



177. Darwin, in his Descent of Man, reasoned circum- 

 stantially that man was essentially an animal, because in in- 

 numerable respects he resembles animals. Proofs in behalf of 

 this thesis he offered in profusion, inquiring into every con- 

 ceivable character which was alleged to be distinctive of man. 

 Throughout, he proceeded on the assumption that differences 

 of degree are of secondary importance. Yet by consistently 

 pursuing such a method he would have experienced no difficulty 

 in proving that all animals are plants, and possibly that no 

 division exists between living and non-living things. 



The truth, however, is that degrees frequently indicate quali- 

 tative differences. E.g., a black piece of iron, as it is being 

 heated, grows successively red-hot and white-hot ; a certain 

 degree of friction produces a spark, and only at a certain stage 

 does chemical combination or decomposition take place. More 

 marvellous still, when the rate of oscillation of electrons is 

 very high, they emit rays which cause the sensation known 

 to us as light, and if "they oscillate even faster than required 

 for this effect, they produce rays of invisible light. Slower 

 oscillations produce rays of heat, and still slower frequencies 



