THE INTRODUCTION OF NEW SPECIES, 3 
cent formations, 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 creation 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 geo- 
graphical distribution 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 explaining them 
all, and giving a direction to new researches. It is 
about ten years since the idea of such a law sug- 
gested itself to the writer of this essay, and he has 
since taken every opportunity of testing it by all 
the newly-ascertained facts with which he has become 
B 2 
