THE INTRODUCTION OF NEW SPECIES. 21 



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 exuviae 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 forma- 

 tion 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 dis- 

 tricts 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 



