THE COINCIDENT DISTRIBUTION OF RELATED 

 SPECIES OF PELAGIC ORGANISMS AS ILLUS- 

 TRATED BY THE CHJi:TOGNATHA. 

 CHARLES ATWOOD KOFOID 



No small part of the diversification of the organic world has 

 taken place in the open sea. Whether we accept the view that the 

 littoral and abyssal faunas are derivatives of the pelagic, or regard 

 the latter as secondarily derived along many lines from the organ- 

 isms of the shore and bottom, the fact remains that many groups 

 have undergone great diversification both in the specific and in 

 higher categories in the pelagic habitat. Illustrations of this 

 process are to be found in the diatoms, the Protozoa (notably 

 the Foraminifera, Radiolaria, Dinoflagellata and Tintinnoina), 

 in the Scyphomedusa^, Siphonophora, and Ctenophora, Ostra- 

 coda, Schizopoda, Amphipoda, Decapoda, Heteropoda, Ptero- 

 poda, Cephalopoda, and Tunicata and certain families of fishes. 

 The Nemertini, Aimelida, Rotifera. Holothuroidca and the llcni- 



are exclusively marine and ])e]agic, and their aflinitirs arc with 

 the more primitive types of invertebrates. It seems })r()l)able 

 that their entire evolution, or at least their generic and si)ecific 

 differentiation has taken place in the marine habitat. Their 

 present distribution is therefore of prime interest because of its 

 bearing on the relation of isolation to the origin and ))reservati()n 

 of species. 



Barriers are far less in evidence in the environment of the j)ehigic 

 fauna than in that of the shore or of the land. A t\nv instances in 

 limited regions along the margins of great ocean currents as, for 

 example, along the edges of the Gulf Stream or in liorizontally 

 stratified waters, there are abrupt transitions in temperature, 

 but in the n)ain t]w changes in temperature, ilhnninution, density, 

 and substances in solution or suspension, are so gra<hial that 

 zoological })rovinces are delimited with difficulty and mainly in 

 terms of tempcratiu-e, on the high seas away from the infiuence of 



