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THE POPULAR SCIENCE MONTHLY. 



rying the line back to the Eocene for- 

 mations, and connecting the present 

 Equus or horse-tribe with an early Eo- 

 cene animal known as the Orohippus. 

 In his lecture Prof. Huxley traced the 

 relationship of these six ancestral forms 

 of the existing horse, and based his argu- 

 ment for the demonstrative evidence of 

 evolution on the continuity and extent 

 of the series. But he went further, and 

 stated what the characteristics of a still 

 earlier form would be if it were ever 

 discovered ; and, within a month from 

 his departure from the country, Prof. 

 Marsh announces that fossils of the 

 predicted animal have been actually 

 found in the lowest Tertiary deposits 

 of the West, giving the Eohippus as the 

 seventh term of paleontological ances- 

 try of the Equine group. 



We pointed out last month that 

 proof is a thing of degrees, and that 

 demonstration may be cumulative ; and 

 the very case we were considering now 

 furnishes further illustration of it. 

 Prof. Huxley says that the doctrine of 

 evolution and the Copernican theory 

 of the motions of the heavenly bodies 

 have precisely the same basis, that is, 

 " the coincidence of observed facts 

 with theoretical requirements ; " and 

 that " an inductive hypothesis is said 

 to be demonstrated when the facts are 

 shown to be in entire accordance with 

 it." But the demonstration becomes 

 still stronger when the requirements of 

 theory lead to the prediction of what 

 must follow from it, and Nature sub- 

 sequently furnishes the facts that vin- 

 dicate the prophecy. It is one of the 

 highest tests of the truth of a theory, 

 that it leads to new discoveries, as was 

 conspicuously the case with the wave- 

 theory, of light. A scientific professor 

 is reported to have said that the proof 

 of the evolution theory is far less strong 

 than that of the undulatory theory, 

 while nobody regards that as demon- 

 strated. On the contrary, it is so re- 

 garded, and with abundant reason. The 

 objective existence of the ether may 



not be proved, but this conception is 

 not essential to the theory, and is held 

 by many as nothing more than a con- 

 venient assumption or hypothetical ar- 

 tifice to aid the imagination in picturing 

 wave-actions. The essence of the the- 

 ory, whether the medium assumed be 

 ethereal or material, is that light origi- 

 nates in some kind of undulatory mo- 

 tion, and a rational optical science is 

 now only possible on this view. In the 

 sense in which Huxley uses the term 

 demonstration, as " the coincidence of 

 observed facts with theoretical require- 

 ments," it is an established demonstra- 

 tion, and evolution stands exactly on 

 the same ground. The facts are what 

 the theory requires them to be, and 

 what it predicts them to be ; it explains 

 them by the operations of real causes, 

 and offers the only explanation we can 

 have without going outside of Nature 

 to get it. 



Prof. Huxley has done us great ser- 

 vice by going over the question of evi- 

 dence in his three lectures, and bring- 

 ing out the full force of the proof for 

 this doctrine, and to make any less 

 claim than he has made is to be want- 

 ing in fidelity to the truth. 



And from this point of view we 

 must think that Prof. Martin, in his 

 admirable introductory discourse, did 

 not fairly represent the case in giving 

 the scientific status of the principles of 

 the conservation of energy and of natu- 

 ral selection. He said : " These ideas 

 may or may not be true ; increase of 

 knowledge may confirm or may pos- 

 sibly upset them." So sharp an alter- 

 native as true or false, determinable 

 only in a contingency of the future, 

 certainly does injustice to the logical 

 validity of these great ideas. They can 

 no more be subverted or abandoned in 

 the future than any other truths of ex- 

 periment and observation. They may 

 disappear by absorption into larger 

 truths, may change aspects, but they 

 are basal and permanent factors of sci- 

 ence, and we see not why the professors 



