COELACANTH IISHES — WHITE 355 



more or less orderly sequence. The coelacanths are first found near 

 the middle of the sequence, some 300 million years ago, and it may 

 help us to appreciate the vastness of this ancestral record when we 

 recall that man did not appear for another 299 million years, for the 

 earliest known remains accredited to the genus Homo are rather less 

 than one million years old. 



From the discovery of their fossil remains we know approximately 

 when each type appeared and how long it persisted. The fossils do 

 not give us the time in years; that is done by physical methods, such 

 as the lead ratio in radioactive minerals whose rate of disintegration 

 is known; but once that has been fixed for a particular series of rock, 

 then the fossils allow us to correlate it with other series with similar 

 faunas or floras elsewhere. Thus fossils do allow us to identify and 

 date the rocks in which they are found. 



But our knowledge of the succession of life as applied to each indi- 

 vidual type of animal or plant is empirical, the result of experience 

 and something that cannot be calculated. We cannot foretell, for 

 instance, how long any newly discovered form will continue, and we 

 do not know why some creatures have persisted through countless 

 ages, sometimes with little change, while others have made a brief 

 and widespread appearance in the geological record, only to disappear 

 as suddenly as they came. It is this last type that is most useful to 

 the geologist, for they can be used to identify a particular stratum 

 and so enable him to make precise correlations of strata over large 

 areas. 



The causes of extinction in nature are not well understood. Many 

 animals and plants, of course, did not actually die out, but instead 

 changed into more advanced forms by the process of evolution. Others 

 undoubtedly did become extinct, and sometimes whole groups like 

 the great reptiles, the dinosaurs, faded out. 



A common explanation of extinction in animals is that with limited 

 food supplies the more primitive types were unable to compete with 

 the more highly developed ; in some cases the latter may have actually 

 preyed upon them, especially their young, as the early mammals are 

 said to have preyed on the untended eggs of the great herbivorous 

 dinosaurs — and of course when they became extinct, then their flesh- 

 eating relatives that fed on them had to disappear also. But earth 

 movements, resulting in the gradual rising or sinking of the land, 

 must have been a prime factor ; for the changes in the level of the land 

 surface may well have affected the climate adversely, and that in 

 turn would affect the animals (especially cold-blooded animals like 

 reptiles) or their food supplies. The possibility of epizootic diseases 

 must also be considered. On more than one occasion the big game 

 of Africa has been decimated over wide areas by rinderpest, and such 



