THE AGE OF THE HABITABLE EARTH 287 



depends upon the amount of sodium present in solution in sea- 

 water. If we knew the total amount of sodium present in the 

 ocean, and if we also knew the rate at which it has been accu- 

 mulating since the ocean was first formed, it would be a simple 

 matter to calculate the age of the ocean and therefore that of 

 the stratified rocks, whose existence must have commenced with 

 that of the ocean itself. The greater part at any rate of the 

 salt contained in the sea must have been derived from the 

 disintegration of the land and carried down by rivers. Estimates 

 have been made, based upon careful analysis, of the amount of 

 sodium contained in the sea and in the water of various rivers, 

 as well as of the amount of water which flows into the sea 

 annually, and Professor Sollas has come to the conclusion that 

 " on a review of all the facts, the most probable estimate of the 

 age of the ocean would appear to lie between 80 and 150 millions 

 of years," which is considerably in excess of the estimate which 

 he arrives at from the consideration of the thickness of the 

 sedimentary rocks. 



We cannot assume, however, that the earth has been habitable 

 during the whole of this period, for when the ocean was first 

 formed both it and the land must have been at far too high a 

 temperature to admit of the existence of protoplasmic organisms. 

 Possibly if we assume 100,000,000 years as the total age of the 

 habitable earth, and therefore as the time which has been available 

 for the evolution of the organic world, we shall be as near as we 

 can get to the truth in the present state of our knowledge. 



We - must now turn to the consideration of the geological 

 record itself, and in the first place we may ask, what is to be 

 legitimately expected from such a record ? Have we any right 

 to expect anything like a complete representation, by fossil 

 remains, of the past history of the organic world ? Obviously 

 the answer to this question must be a decided negative. The 

 imperfection of the geological record is due to many causes. It 

 must have been imperfect when first laid down, for the great 

 majority of organisms usually disintegrate and pass away 

 without leaving any recognizable remains behind them at all. 

 In most cases it is only hard skeletal structures that have any 

 chance of being preserved. Bodies composed entirely of soft 

 tissues, such as jelly-fish and many worms, are rarely repre- 

 sented by fossils. Even hard structures, such as animal 

 skeletons, will only be fossilized if they happen by some 



