feet of men. Fossils were known to the Ionian Greeks in the sixth century b.c., they were discussed at Alexandria by Eratosthenes and others in the third century b.c., a discussion which is summarized in Strabo's Geography (? 20-10 b.c.). They were known to the Latin poet Ovid, but he did not understand their nature. He thought they were the first rude efforts of creative power. They were noted by Arabic writers in the tenth century. Leonardo da Vinci, who lived so recently as the opening of the sixteenth century (1452-1519), was one of the first Europeans to grasp the real significance of fossils, and it has been only within the last century and a half that man has begun the serious and sustained deciphering of these long-neglected early pages of his world's history.
Speculations about geological time vary enormously. Estimates of the age of the oldest rocks by geologists and astronomers starting from different standpoints have varied between 1,600,000,000, and 25,000,000. The lowest estimate was made by Lord Kelvin in 1867. Professor Huxley guessed at 400,000,000 years. There is a summary of views and the grounds upon which the estimates have been made in Osborn's Origin and Evolution of Life; he inclines to the moderate total of 100,000,000. It must be clearly understood by the reader how sketchy and provisional all these time estimates are. They rest nearly always upon theoretical assumptions of the slenderest kind. That the period of time has been vast, that it is to be counted by scores and possibly by hundreds of millions of years, is the utmost that can be said with certainty in the matter. It is quite open to the reader to divide every number in the appended time diagram by ten or multiply it by two; no one can gainsay him. Of the relative amount of time as between one age and another we have, however, stronger evidence; if the reader cuts
- Theophrastus, quoting Xenophanes.
- There is a discussion of fossils in the Holkham Hall Leonardo MS.
- An admirable recent book, short and written in a style intelligible to the general reader, is Arthur Holmes, The Age of the Earth. He gives a good summary of this most interesting discussion, and sustains the maximum estimate of 1600 million years.