472 THE POPULAR SCIENCE MONTHLY. 



could furnish us with as infallible a prediction applicable to 1980 or 

 2980. 



But astronomy is not less remarkable for its power of retrospective 

 prophecy. 



Thales, oldest of Greek philosophers, the dates of whose birth and 

 death are uncertain, but who flourished about 600 b. c, is said to have 

 foretold an eclipse of the sun which took place in his time during a 

 battle between the Medes and the Lydians. Sir George Airy has writ- 

 ten a very learned and interesting memoir * in which he proves that 

 such an eclipse was visible in Lydia on the afternoon of the 28th of 

 May, in the year 585 b. c. 



No one doubts that, on the day and at the hour mentioned by the 

 Astronomer Royal, the people of Asia Minor saw the face of the sun 

 totally obscured. But, though we implicitly believe this retrospective 

 prophecy, it is incapable of verification. It is impossible even to con- 

 ceive any means of ascertaining directly whether the eclipse of Thales 

 happened or not. All that can be said is, that the prospective prophe- 

 cies of the astronomer are always verified ; and that, inasmuch as his 

 retrospective prophecies are the result of following backward the 

 very same method as that which invariably leads to verified results 

 when it is worked forward, there is as much reason for placing full 

 confidence in the one as in the other. Retrospective prophecy is 

 therefore a legitimate function of astronomical science ; and if it is 

 legitimate for one science it is legitimate for all ; the fundamental 

 axiom on which it rests, the constancy of the order of nature, being 

 the common foundation of all scientific thought. Indeed, if there can 

 be grades in legitimacy, certain branches of science have the advan- 

 tage over astronomy, in so far as their retrospective prophecies are 

 not only susceptible of verification, but are sometimes strikingly 

 verified. 



Such a science exists in that application of the principles of biology 

 to the interpretation of the animal and vegetable remains imbedded in 

 the rocks which compose the surface of the globe, which is called pale- 

 ontology. 



At no very distant time, the question whether these so-called 

 " fossils " were really the remains of animals and plants was hotly 

 disputed. Very learned persons maintained that they were nothing of 

 the kind, but a sort of conci'etion or crystallization which had taken 

 place within the stone in which they are found ; and which simulated 

 the forms of animal and vegetable life, just as frost on a window-pane 

 imitates vegetation. At the present day it would probably be impos- 

 sible to find any sane advocate of this opinion ; and the fact is rather 

 surprising that among the people from whom the circle-squarers, per- 

 petual-motioners, flat-earth men, and the like, are recruited, to say 



* " On the Eclipses of Agathocles, Thales, and Xerxes," " Philosophical Transactions," 

 vol. cxliii. 



