194 Mr. A. R. Wallace on the haw which has regulated 



doubted; but we should compare their numbers not merely with 

 those that now exist upon the earth, but with a far larger 

 amount*. We have no reason for believing that the number of 

 species on the earth at any former period was much less than at 

 present; at all events the aquatic portion, with which geologists 

 have most acquaintance, was probably often as great or greater. 

 Now we know that there have been many complete changes of 

 species ; new sets of organisms have many times been introduced 

 in place of old ones which have become extinct, so that the 

 total amount which have existed on the earth from the earliest 

 geological period must have borne about the same proportion to 

 those now living, as the whole human race who have lived and 

 died upon the earth, to the population at the present time. 

 Again, at each epoch, the whole earth was no doubt, as now, 

 more or less the theatre of life, and as the successive generations 

 of each species died, their exuvige and preservable parts would 

 be deposited over every portion of the then existing seas and 

 oceans, which we have reason for supposing to have been more, 

 rather than less, extensive than at present. In order then to 

 understand our possible knowledge of the early world and its 

 inhabitants, we must compare, not the area of the whole field of 

 our geological researches with the earth^s surface, but the area 

 of the examined portion of each formation separately with the 

 whole earth. For example, during the Silurian period all the 

 earth was Silurian, and animals were living and dying, and 

 depositing their remains more or less over the whole area of the 

 globe, and they were probably (the species at least) nearly as 

 varied in different latitudes and longitudes as at present. What 

 proportion do the Silurian districts bear to the whole surface of 

 the globe, land and sea (for far more extensive Silurian districts 

 probably exist beneath the ocean than above it), and what portion 

 of the known Silurian districts has been actually examined for 

 fossils ? Would the area of rock actually laid open to the eye 

 be the thousandth or the ten-thousandth part of the earth's 

 surface ? Ask the same question with regard to the Oolite or the 

 Chalk, or even to particular beds of these when they differ consi- 

 derably in their fossils, and you may then get some notion of 

 how small a portion of the whole we know. 



But yet more important is the ])robability, nay, almost the 

 certainty, that whole formations containing the records of vast 

 geological periods are entirely buried beneath the ocean, and 

 for ever beyond our reach. Alost of the gaps in the geological 

 series may thus be filled up, and vast numbei-s of unknown and 

 unimaginable animals, which might help to elucidate the affinities 

 of the numerous isolated groups which are a perpetual puzzle to 



[* See on this subject a paper by Professor Agassiz in the ' Annals ' for 

 November 185-1. — Ed.] 



