No. 567] EFFECT OF DISTRIBUTION ON SPECIATION 147 



genera in successively larger areas, the index of modifica- 

 tion increasing from 1.40 in the small island of Bourbon 

 to 1.95 in Madagascar, and 3.45 in Africa. Combining 

 Africa and Madagascar, this is increased to 3.67, and with 

 the island of Bourbon, to 3.71. 



Table XIII is one of especial interest, since it deals 



TABLE XIII 



Speciation of Marine Gammaridea (Amphipoda) in Various Seas 

 Data from Stebbing (1906) 



with a marine instead of a terrestrial group. It embodies 

 the results of a compilation of the marine genera and 

 species of Amphipoda of the suborder Gammaridea in a 

 number of the oceans and seas of the world. Since it is 

 primarily a cold-loving group, the largest numbers are 

 found in the cold seas, the Arctic and North Atlantic being 

 the home of considerably over half of the known marine 

 species. It is very likely that when the Antarctic regions 

 have been studied as thoroughly as the northern regions, 

 the number of species from that part of the world will be 

 very considerably increased. At the time of Stebbing 's 

 work on Amphipoda, our knowledge of Antarctic and 

 contiguous areas was very meager. 



The steady increase of the index of modification from 

 the smaller to the larger seas is striking. The Mediter- 

 ranean Sea, although it is the most thoroughly known of 

 all, has the lowest index of modification, namely 2.19, the 

 Arctic Ocean comes next with 2.22, and then the North 

 Atlantic with 2.70. The small number of species from the 

 South Atlantic and Antarctic regions has already been 



