,^. NOTES AND COMMENTS. 487 



The Age of the Earth. 



About the time when Sir Archibald Geikie's Presidential 

 Address to the British Association was exciting interest in the ever- 

 recurring question of the Age of the Earth, the Physical Society of 

 Glasgow University issued an admirable address by Professor John 

 Young on the same subject. Both authors approached the problem 

 from the point of view of the Naturalist as opposed to that of the 

 Physicist ; and both alluded to the circumstance that, whereas the 

 "mathematical mill" will satisfactorily "grind whatever is put into 

 it," no certainty can be reached so long as the initial data are merely 

 plausible assumptions. While, however, Sir Archibald Geikie finds 

 it impossible to accept Lord Kelvin's low estimate of geological time, 

 the Physicist's colleague at Glasgow points out that there are many 

 possibilities of grave error in the biological and geological arguments 

 which may, after all, account for the difference of the result based 

 upon the two hnes of reasoning. The enormous period of time 

 required for the evolution of the present world of life according to 

 current views of Biology is. Professor Young thinks, a hopeless 

 exaggeration due to our ignorance of the production of new organisms 

 by some mysterious sudden change of the old ones. Following 

 Eimer, he especially cites the case of the existing Axolotl, which 

 remains a gill-breather so long as it can live in water, but suddenly 

 changes into the lung-breathing Amblystoma as soon as water is in- 

 accessible. He also alludes to the crustacean Arieiiiia, a denizen of 

 brine springs, which undergoes specific, even generic variation, 

 according to the salinity of the water. Indeed, the Professor goes 

 still further and declares himself in favour of the heresy, that the 

 same type of animal may be simultaneously evolved from a distinct 

 set of ancestors in two remote regions, thus doing away with the 

 necessity of assuming great periods of time for the accomplishment 

 of certain migrations. The oft-quoted arguments of the geologist 

 also fare badly in Professor Young's hands. He points out that the 

 rate of denudation of a country depends so much upon varying con- 

 ditions as to render even plausible results impossible ; and when 

 a geologist speaks of strata miles in thickness, the Professor adds a 

 word of caution that we fear has not been sufficiently attended to by 

 those who attempt to estimate the length of geological periods. 

 Professor Young indicates the great difficulty of determining the 

 contemporaneity of deposits formed under different conditions, and 

 he thinks that the estimated thickness of many geological formations 

 has been greatly exaggerated by piling contemporaneous lake 

 deposits, shore deposits, and deep-water deposits one upon the other. 

 Finally, we are warned that some of our cherished views as to the 

 antiquity of man may be the unfortunate outcome of the independence 

 of the several lines of research, which have never been carefully 

 correlated : it may be that " the net result is the expression not of 

 conviction but of mutual courtesy." 



