I 



the Introduction of New Species, 185 



Now, taking this as a fair statement of the results of geolo- 

 gical inquiry, we see that the present geographical 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 inha- 

 bitants. Many causes no doubt have operated of which we must 

 ever remain in ignorance, and we may therefore 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 suggested itself to the 

 writer of this paper, and he has since taken every opportunity of 

 testing it by all the newly ascertained facts with which he has 

 become acquainted, or has been able to observe himself. These 

 have ail served to convince him of the correctness of his hypo- 

 thesis. Fully to enter into such a subject would occupy much 

 space, and it is only in consequence of some views having been 

 lately promulgated, he believes in a wrong direction, that he 

 now ventures to present his ideas to the public, with only such 

 obvious illustrations of the arguments and results as occur to 

 him in a place far removed from all means of reference and 

 exact information. 



The following propositions in Organic Geography and Geology 

 give the main facts on which the hypothesis is founded. 



Geography, 



1. Large groups, such as classes and orders, are generally spread 

 over the whole earth, while smaller ones, such as families and 

 genera, are frequently confined to one portion, often to a very 

 limited district. 



2. In widely distributed families the genera are often limited in 



range; in widely distributed genera, well-marked groups of 

 species are peculiar to each geographical district. 



3. When a group is confined to one district, and is rich in spe- 



cies, it is almost invariably the case that the most closely 

 allied species are found in the same locality or in closely ad- 

 joining localities, and that therefore the natural sequence of 

 the species by affinity is also geographical. 



4. In countries of a similar climate, but separated by a wide sea 

 or lofty mountains, the families, genera and species of the 



Ann. ^f Mac/. N. Hist. Ser. 2. Vol. xvi. 13 



