is set forth, all evidence adding to the strength of its inferences, is a 

 verification of it ; for the facts of evidence were implied to exist by the 

 theory, they being merely the manifestations of the thing which the 

 theory affirms to exist. The verification of a theory of definition, then, 

 consists in revealing the facts affirmed by it in experience ; or the im- 

 plications of those facts. 



How are theories of origin verified? Darwin's theory is a theory 

 of origin. The conclusion drawn in the theory is that species origi- 

 nated by virtue of individual variations, inheritance, and natural selec- 

 tion. Here, we cannot directly show this to be true by reproducing the 

 past history of organic life, as we did show matter to be electrical by 

 having it manifest the qualities of electricity. We can, however, show 

 by experiment that individual variations, inheritance, and selection 

 will produce species. We can exhibit this fact in experience. But 

 the further verification of the theory requires us to show that these 

 forces did actually so operate in the past as to produce present 

 species. If this was a fact, certain things are implied to be true in 

 the present, and the verification of the theory will consist in revealing 

 in experience the truth of these implications. If the forces of evolution 

 have so operated in the past, the remains of organic life will, where 

 preserved, show the gradual variation in type, fitting the succeeding 

 generations better for life. When such conditions of the remains of 

 organic life are shown, the validity of the theory is to some extent evi- 

 denced or verified. Thus the verification of a theory, whether it be 

 one of definition or of origin, consists in revealing in experience the 

 facts affirmed by the theory to be true; or the implications of those 

 facts. Therefore, in so far as the materials of a theory can be sub- 

 jected to observation or experiment, the theory is verifiable. Either the 

 facts affirmed by the theory must be revealed, or the implications made 

 manifest. Those theories founded upon the experimental method have 

 already revealed many of the implications of the facts inferred, in the 

 evidence gathered for the theory. All that stands in the way of fur- 

 ther verifying such theories is the improvement of the facilities for ex- 

 perimenting, and the application of these facilities. 



Experiment will also be the method of verifying theories which 

 make use of assumptions. The theory of ions assumes the atom to be 

 constructed in a certain way. The theory can be verified in this re- 

 spect by showing that the implications of this assumption are true. 

 If this assumption is true, the atomic weights of atoms will change 

 in accordance with a certain law. By experiment, it is shown that the 

 atomic weights do change according to that law. In this way the as- 

 sumption is to a certain extent verified. The assumption may be fur- 

 ther verified either by a direct examination of the atom, if that be- 



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