16 NATURAL SELECTION 1 
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 geo- 
logical 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 deposit: 
ing 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 considerably 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 probability, 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. Most of the gaps in the 
geological series may thus be filled up, and vast numbers of 
unknown and unimaginable animals, which might help to 
elucidate the affinities of the numerous isolated groups which 
are a perpetual puzzle to the zoologist, may there be buried, 
till future revolutions may raise them in their turn above the 
waters, to afford materials for the study of whatever race of 
intelligent beings may then have succeeded us. These con- 
siderations must lead us to the conclusion that our knowledge 
of the whole series of the former inhabitants of the earth is 
