PREFACE. 



Amongst the Reptiles and Amphibians obtained by Mr. Darwin, in the Voyage 

 of the Beagle, there are several of great interest, not merely on account of their 

 novelty as newly discovered species, of which there are nearly thirty, or as 

 forming the types of genera not previously known, or of any remarkable pecu- 

 liarity of form, structure, or habit, although in all these respects many of them 

 are highly interesting ; but more particularly as serving to establish or confirm 

 several points connected with their geographical distribution. 



From the structure of most of these animals and their consequent habits of 

 life, circumscribed as they are for the most part in their locomotive powers, it 

 might reasonably be predicated that they would, upon the whole, exhibit as 

 distinct examples of restriction, with regard to their geographical boundaries, 

 as any class of vertebrated animals ; and that the intervention of seas and of 

 mountains would be sufficient to limit the range of a species. Such is in fact 

 usually the case ; and not only is the same species not found in the Old and New 

 Continents, but, with very few exceptions, not even on the opposite sides of the 

 South American Continent, in which range Mr. Darwin's discoveries have 

 principally been made. The occurrence, however, of Biifo Cliilensis at Rio 

 Janeiro and at Buenos Ayres on the eastern, and at Valparaiso and the Archipe- 

 lago of Chonos on the western side of the continent, shows an extent of dis- 

 tribution exceedingly unusual if not absolutely unparalleled in this family. It is, 

 however, still possible that further and more extended researches into the 

 characters of the animals in question, and an examination of individuals from 

 each locality at various ages, may prove that there are two species, which have 

 been confounded with each other, and the anomaly may thus be removed. 



But although the circumscribed range of a species may be accounted for by 

 the reasons above mentioned, and others of a restrictive nature, it is not so easy 

 to refer to any known or obvious cause the remarkable fact of a whole genus, 

 consisting of numerous species, being thus geographically limited. Yet this is a 



