258 ANNUAL REPORT SJVnTHSONIAN INSTITUTION, 19 40 



the rate of cooling of cast-iron balls, and from the results boldly 

 stated that the six days of Biblical creation were "periods" to be 

 extended as the facts required, and that the age of the earth was 

 actually some 75,000 years. 



Scotland took a prominent part in the many discussions of the 

 subject which took place in the nineteenth century. It was Lord 

 Kelvin who, from the secular cooling of the earth, concluded that 

 the globe must have consolidated not less than 20 millions of years 

 ago, and finally suggested limits between 20 and 40 millions of years; 

 and it was Professor Tait who reduced that estimate to from 10 to 

 15 millions of years. The geologic view was expounded by Sir 

 Archibald Geikie at meetings of the British Association at Dover 

 in 1899 and at Edinburgh in 1892, and he summed up for an interval 

 of "probably not much less than 100 million years since the earliest 

 forms of life appeared upon the earth and the oldest stratified rocks 

 began to be laid down." 



From these earlier discussions it would appear that a psychological 

 element entered into the final estimates, as if the calculators drew 

 back aghast at the possibility of the enormous age of the earth at 

 which their estimates hinted. Thus almost all tended in their final 

 sunmiing toward the minimum of their scales, and little is heard 

 of the other extreme — Lord Kelvin's independent maxima, reached 

 by different methods, of 1,000, 400, and 500 million years, or Geikie's 

 400 million years — although these came much nearer to the modem 

 astimate. 



The effect upon the biologist of the varying estimates, of their 

 uncertainty and the adverse criticism to which they were subjected, 

 was such that he simply ignored them. Thirty years ago it was 

 the worst possible form, zoologically, to mention them. But now, 

 thanks to a consensus of opinion that admits credibility to estimates 

 based upon the break-up of radioactive minerals in the rocks, the 

 reputed ages of the geologic formations are creeping, somewhat 

 apologetically, into the biological textbooks. They have passed 

 tlirough the textbook needle's eye ; that is a sign for anyone to read. 



Some of you will remember the joint discussion, shared in by the 

 Zoology Section, at the meeting of the British Association in Edin- 

 burgh in 1921, when Lord Rayleigh summarized the physical evi- 

 dence from the break-down products of uranium. Since then the 

 methods of estimation and calculation have been refined, but the 

 results of several methods appear to be in substantial agreement. 

 Thus the beginning of the Cambrian formations, with their well- 

 preserved fossils, were laid down probably about 500 million years 

 ago; below the Cambrian are formations with relatively few and 

 mostly ill-defiaed fossils^ which carry the relics of life back some 



