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THE AMERICAN NATURALIST [Vol. XLI 



we would be forced to conclude that isolation has had no part in 

 the origin, differentiation, and continuance of these related species. 



In Dagitta bipunctata Miss Stevens ( :03) has described a method 

 of close fertilization. As yet we have no light on the extent of its 

 occurrence in other species where the presence of enlarged seminal 

 vesicles and external male parts affords suggestive though not con- 

 clusive evidence of external and presumably of cross fertilization. 

 Should all species of Chsetognatha prove ultimately to have close 

 fertilization we would have in this a most effective means of isola- 



The apparently wide-spread phenomenon of coincident dis- 

 tribution of related species among pelagic organisms appears to 

 cast some doubt upon the universality of the operation of isolation 

 in the evolution of species as originally maintained by IVIoritz 

 Wagner ('68) and recently revived by President Jordan (:05). 



The contrast here afforded also raises the question whether the 

 two types of 'species' really belong fundamentally to the same 

 category or not. Are those with contiguous distribution, and also 

 many of the geographical species and subspecies of land verte- 

 brates, of a standing exactly equivalent to that of those having a 

 coincident distribution? Are, for example, S. furrata and S. 

 planctonis merely the extremes of an environmental series begin- 

 ning in the warm surface waters and ending in deep waters of 

 lower temperature? In other words are they the result, in part 

 at least, of the pressure of the environment ? A statistical study 

 of the distribution and variation of such a pelagic couplet and a 

 comparison with a similar study of a couplet having a coincident 

 distribution would be most instructive in indicating whether or 

 not any distinction exists between 'isolation-environmental' spe- 

 cies on the one hand and 'selection-nuitation (?)' species on 

 the other. Are intermediate forms (Hjiially absent in both types of 



the Llivi.huds of the two types? Ab..vr all will tl„- in.livi.luals oi 



ronnuM.ts are trimsp..>(Ml ? An.l linally will tlir ,>,Hvi,«> with coin- 

 cident <listributi()n exliibit any greater specific ..tal)ility ^un<ler 



Investigators of pelagic organisms have been morphologists so 



