DISPEKSAL OF LIFE^ — VISHEK 319 



is thought by some that the sea level was lowered about GO feet dur- 

 ing the Pleistocene glacial epochs. Unquestionably there was some 

 lowering of the sea level, but probably not nearl}^ that much, for the 

 glaciers probably were not nearly so thick as some have assumed." 

 However, any considerable lowering of the sea would result in an 

 inci'ease in the land area, sometimes connecting neighboring islands, 

 and hence facilitating the local spread of land forms. 



DURATION OF GEOLOGIC TIME 



The recent great increases in the estimates of the duration of the 

 geologic past also has a bearing upon the subject of tropical cyclones 

 and the dispersal of life in the Pacific. It is conceded that the 

 chances of a single hurricane doing mucli along this line are small. 

 The great reason why hurricanes have been largely ignored by the 

 students of this problem is because hurricanes were believed to occur 

 only at long intervals and were thought to be too rare to play much 

 of a part. But now that it is Icnown that more than a score occur 

 annually, on the average, out in the Pacific (in addition to those in 

 the Philippine region) they take on a different aspect. Their sig- 

 nificance is also increased by the fuller appreciation of the diverse 

 ways in which they affect the lands, streams and currents. They are 

 also less likely to be ignored as a factor in the dispersal of life when 

 it is realized that the interval since the beginning of the Mesozoic 

 era probably is many times the 20,000,000 to 50,000,000 years it was 

 thought to be a few decades ago. Indeed, the remarkably convincing 

 evidence, derived by leading physical chemists from the study of the 

 products of the disintegration of radioactive substances in igneous 

 rocks of various geologic ages, indicates that probably more than a 

 billion years have elapsed since life became abundant upon the earth. 

 Hence, fortuitous happenings, such as the dispersal of living forms 

 by hurricanes, have had a long time in which to succeed. 



" Huntington and Visher, " Climatic Changes, tlieir Nature and Causes," New Haven, 

 p. 125, 1922. 



