68 PROCEEDIJfGS OF THE GEOLOGICAL SOCIETY. 



one stock in each case, but that similarity in organic structure is 

 proportional, as a general rule, to the degree of affinity, animals and 

 plants that are like being more nearly related by descent than those 

 which are dissimilar. - 



xis I have already said, I propose to treat the subject of distribu- 

 tion at some length, both because it is, I think, well worthy of the 

 attention of geologists, and because I believe the whole question 

 requires reconsideration. To understand why this is the case, it is 

 essential briefly to recapitulate the history of the inquiry. 



Although much had been previously done, the first contribution 

 to which reference is necessary was a paper by Mr. P. L. Sclater, 

 published in 1858 *, "On the General Geographical Distribution of 

 the Members of the Class AvesJ' The terrestrial area of the world 

 was in this paper divided into the following six zoological regions : — 



1. Palcearctic : Europe, JN'orthern Africa, Northern and Central 



Asia. 



2. Ethiopian : Africa south of the Atlas, and Madagascar. 



3. Indian^ renamed Oriental by Wallace : India, South-eastern 



Asia, and part of the Malay Archipelago. 



4. Australian : Australia, with New Guinea and adjacent islands, 



New Zealand, and Polynesia. 



5. Nearctic : America as far south as Mexico. 



6. Neotropical: Central and South America, with the West 



Indies. 



These regions were founded solely on birds, and mainly on 

 passerine birds (or on passerine and picarian). They were accepted 

 for snakes and Batrachia by Dr. Gilnther in a paper published in 

 1858 t ; but afar more extended study of the subject, and the great 

 additions made to our knowledge within the last thirty years, have 

 induced Dr. Giinther to come to a very different conclusion, as will 

 be shown presently. 



The papers just mentioned appeared before the publication of 

 Darwin's ' Origin of Species,' and were, of course, written without 

 any reference to the idea of relationship by descent amongst different 

 genera of a family or different species of a genus. So soon, however, 

 as Darwin's great work had produced its effect, the importance of 

 an inquiry into the distribution of animal- and plant-life was greatly 

 increased. 



* Journ. Linn. Soc. Zoology, ii. p, 130. 

 t P. Z. S. 1858, p. 373. 



