GEOGRAPHICAL ISOLATION 417 



lizard, Lygosoma moco, is represented on Pitt Island (one of the 

 Chathams) by a very similar form described by Mr. Boulenger 

 under the name Lygosoma dendyi. The remarkable New Zealand 

 lance-woods (Pseudopanax crassi/olium and P. ferox) are repre- 

 sented on Chatham Island by the closely related Pseudopanax 

 chathamicum, and the Chatham Island ribbon-wood also differs 

 slightly from the common New Zealand species (Plagianthus 

 betulinus). 



The explanation of these differences is that the two portions 

 into which each of the species mentioned became divided when 

 the Chatham Islands were separated from New Zealand have 

 diverged from one another and followed somewhat different lines 

 of evolution. Owing, perhaps, to slightly different conditions of 

 the environment, or to other causes which it is impossible to 

 specify, one or both has become modified to a greater or less 

 extent in its own particular direction and, owing to the 

 geographical isolation, there has been no interbreeding between 

 the two sections to keep them both in the same average condition. 



The principle of isolation fully explains why the fauna and flora 

 of oceanic islands in general are made up almost entirely of 

 peculiar species found nowhere else in the world. The ancestors 

 of these species were originally derived from some very distant, 

 probably continental area, and their descendants have had few 

 if any opportunities of interbreeding with the parent species, 

 from which they have gradually diverged further and further 

 under their new conditions of life. 



Certain writers, such as Mr. Gulick and Dr. Romanes, have 

 maintained that the mere separation of a species into two or 

 more sections which are prevented from interbreeding would 

 suffice to bring about divergent evolution, irrespective of whether 

 or not the separate sections were exposed to different environ- 

 mental conditions. It would probably be impossible to divide 

 a species into two sections whose average qualities are identical, 

 and: 



" No matter how infinitesimally small the difference may be 

 between the average qualities of an isolated section of a species 

 compared with the average qualities of the restof that species, if 

 the isolation continues sufficiently long, differentiation of specific 

 type is necessarily bound to ensue." l 



1 Komanes, " Darwin and after Darwin," Vol. Ill, " Isolation and Physiological 

 Selection," p. 13. 



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