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 exuvie 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 
