4 NATURAL SELECTION 1 
during an immense but unknown period the surface of the 
earth has undergone successive changes; land has sunk be- 
neath the ocean, while fresh land has risen up from it ; 
mountain chains have been elevated; islands have been 
formed into continents, and continents submerged till they 
have become islands ; and these changes have taken place, 
not once merely, but perhaps hundreds, perhaps thousands of 
times.— That all these operations have been more or less 
continuous but unequal in their progress, and during the 
whole series the organic life of the earth has undergone a 
corresponding alteration. This alteration also has been 
gradual, but complete; after a certain interval not a single 
species existing which had lived at the commencement of the 
period. This complete renewal of the forms of life also 
appears to have occurred several times.—That from the last 
of the geological epochs to the present or historical epoch, 
the change of organic life has been gradual: the first appear- 
ance of animals now existing can in many cases be. traced, 
their numbers gradually increasing in the more recent forma- 
tions, while other species continually die out and disappear, 
so that the present condition of the organic world is clearly 
derived by a natural process of gradual extinction and crea- 
tion of species from that of the latest geological periods. 
We may therefore safely infer a like gradation and natural 
sequence from one geological epoch to another. 
Now, taking this as a fair statement of the results of 
geological inquiry, we see that the present geographical dis- 
tribution of life upon the earth must be the result of all the 
previous changes, both of the surface of the earth itself and 
of its inhabitants. Many causes, no doubt, have operated of 
which we must ever remain in ignorance, and we may, there- 
fore, expect to find many details very difficult of explanation, 
and in attempting to give one, must allow ourselves to call 
into our service geological changes which it is highly probable 
may have occurred, though we have no direct evidence of 
their individual operation. 
The great increase of our knowledge within the last twenty 
years, both of the present and past history of the organic 
world, has accumulated a body of facts which should afford 
a sufficient foundation for a comprehensive law embracing and 
