246 



THE A M ERICA X NA T 1 1L\ LIST [Vol. XLI 



20° to 32° — 11 



This, in conjunction with the fact that some of the low tempera- 

 ture species belong to the mesoplankton of the tropics, indicates 

 that the center of radiation of the (;enus has been in the tropics, 

 or that specific differentiation has been relatively more rapid in 

 that region than at the lower tenij)eratnres toward the poles. 



These broader outlines of the distril)uti()n of the species of 

 Sagitta are suggestive of a considerable degree of coincidence of 

 distribution of species, it may be of closely related ones, and 

 prompts to a closer analysis of their relationships and distribution. 



The determination of degrees of relationship among species of a 

 genus is a matter of inference from structural details for whose 



guided by the selection of characters on whicli chissification is 

 based, l>y experience in deahng with the specific analysis of the 

 material, and subjectively, by the conception of sjx'cies which one 



modified structures, some clue t.^ the distance of their removal 

 from the i)arent stock, or from each other. On the other hand 

 the elementary >pecie> arising by nuUation from <Knnthrra Immirvk- 

 iana mav be regarde.l a> geneti<-ally e<|nally related t.> each other 

 or to the parent .to<-k, but if we bas,> oiu' judgment of the degrees 

 of the relationship which they exhibit solely on the structural 

 characters whi. li di.tinguidi them, wc wonM be for.vd to conclude 

 that there wa. considerable disparity of relationship among them. 

 The n.ntation tlicry admit, a wid.^r latitn.i.' in <'sti,naling the 

 relati<.n.hip of .pccic. than docs tlu- unmodiiic.l Darwinian point 



We have, however, in Sagitta onlv the ronit, and n..t the process 

 of si>ecific .litrerentiati<.n with which to deal, and are therefore 



