SECT. 4] BIOLOGICAL SPECIES, WATER-MASSES AND CURRENTS 403 



basis of morphometric characteristics (ratios of measurements of the eye and 

 abdomen) (Brinton, 1962b). They appear to be adapted to separate geographical 

 areas and make up a Rassenkreis along the anticyclonic pathway of the circula- 

 tion of the subtropical and tropical North Pacific. The biogeographical sequence 

 includes a "California Current" form, an "Eastern Equatorial" form, a "Western 

 Equatorial" form, and a "Central" form. None are regarded as species because 

 each appears to intergrade with neighboring upstream or downstream forms. 

 Thus, they are sub-species, in the sense that the bulk of the population of each 

 form is morphologically uniform. However, the intergrades have been found 

 only in small parts of the overlapping ranges. The "California Current" and 

 "Central" forms overlap, but are regarded as terminal elements of the cline 

 because they do not intergrade with each other. 



In another complex, Euphausia nana, occupying the Tsushima water-mass 

 of the East China Sea, is more nearly isolated from E. pacifica, its large sub- 

 arctic counterpart (Brinton, 1962a). E. pacifica, ranging eastward from the 

 Japan Sea to the California Current (Fig. lb), differs conspicuously from E. 

 nana in that it is about three times larger at maturity. Morphological dif- 

 ferences are small. Though these differences might have ecotypic bases, the 

 absence of intergrading specimens has led to the assignment of specific status 

 to both forms. The partial barriers of the Tsushima Strait and the Kuroshio 

 may be insufficient to have brought about genetic isolation of the two popula- 

 tions ; however, it appears possible that unequal maturation rates associated 

 with the different water-mass habitats may have reproductively isolated the 

 large northern adults from the small southern ones. On the other hand, eggs of 

 the large form, after drifting into the southern habitat, might develop into 

 small adults, thereby sustaining a uniform E. pacifica-nana gene pool. 



It is not possible at present to provide a satisfactory solution of these com- 

 plex relationships in terms of oceanographic factors. Environmental gradients 

 exert continuing, however variable, pressure on the adaptive capacity of all 

 plankton populations. The populations are thereby constantly subject to both 

 physiological and genetic adaptations, but the implications of these to species 

 formation are clear only when gene pools that are not in communication with 

 each other have been created. 



3. Water Currents and Biological Species 



Water-masses are conceived of as being sufficiently large in volume to 

 maintain rather steady and uniform conditions throughout circumscribed 

 geographic areas. When these masses are maintained as such over large areas 

 or are confined as semi-closed systems of circulation, a characteristic assemblage 

 of planktonic species may develop. However, these waters are constantly being 

 formed anew by admixtures of water from other systems, each with their own 

 identity. These blended waters, although maintaining their physical-chemical 

 identity for the main body, are changed at their margins and extremities by 

 admixture of yet other waters, or by physical changes resulting from seasonal 



