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 lie has become 



B 2 



