No. 484] DISTRIBUTION OF ( U.KK u.S.\ TIlS 249 



The members of the first couplet, /urco/r/ i>!(iiic/>>n is. a iv Atlantic 

 species, the former of wide distribution, '>\ X. to 7 S., in the 

 epiplankton of colder waters (17°) and the nicsoplankton of the 

 tropics. The latter occurs only in the epiplankton of the tro|)ics. 

 This couplet of most closely related species has a contiguous 

 rather than a coincident distribution. The distribution of both, 

 however, is overlapped by that of the very closely related S. hi- 

 punctata. The degrees of relationship as suggested by quantita- 

 tive characters may be inferred from the accompanying table. 



regularis 4.5-7 28-40 5-7 2-4 4-6 



The members of the second couplet of most closely related 

 species, S. regularw and SI. ncglecta, are both surface neritic forms 

 of the Malay Archipelai^o and Japanese waters. .S'. rrr/u/an's is 

 neritic also about tlie MaMives and ir may !)<■ that 1 )<>iicaster (:03) 

 overlooked the very similar N. iiKjlicfn in the eoUeetions from 

 these waters. The distribution of these t\v.. most cIom Iv related 

 species is thus widely overlapping, if not in(h-ed eoiiK ident. 



The distribution of pelagic organisms, a- ilhisrrated by the 

 Chtetognatha thus affords several probable in>taiices ,,f the isola- 

 tion of the members of couplets of most closely related ^iieeies hy 

 isotherms or isothermobaths. This isolation is similar in many 

 of its aspects to that so often foimd hetweeti terrestrial speci.^s. 

 It may well be that isolation has heen an essential factor in the 

 differentiation of the members of these couplets. l-:\en more 

 general, however, in the pelagic world and amonu- the species of 



We have now no evidencr of 'ditrere.uial s.-as,,,',.. temper '.lures, 

 or levels at which breedin- mi-ht occur in thes,> closely related 

 species. Should these ditferentials ultimately prove to be absent 



