124 University of California Publications. [Zoology 



elements which the genus presents, which may occur as addi- 

 tional species, and it requires no gift of prophecy to predict that 

 continued exploration of the sea will bring to light additions to 

 the genus. 



If then natural selection does not originate the specific char- 

 acters of Triposolenia we are forced to seek some other explana- 

 tion of the complex of structures found in the organization of 

 each species. The mutation theory of De Vries ( :05) affords a 

 conception of the origin of these species which avoids the diffi- 

 culties attendant upon the absence of differential survival values 

 of the specific characters and also lends itself readily to the view 

 here advocated that specific distinctions involve not a few but 

 many characters of the organism. 



///. TJic Coi)icidc)it Distribution of Belated Species of 

 Triposolenia. 



The data available for the critical determination of this ques- 

 tion are inadequate at present. In the first place the individuals 

 are always relatively rare, 1-6 representing the number of indi- 

 viduals usually found in a haul of a 14-inch net. Again, closing 

 net catches have not been examined in sufficient numbers and 

 from sufficient localities to determine the vertical distribution of 

 the species. It is possible that there may be a differential vertical 

 distribution and that temperatures may form barriers between 

 the species. I have no conclusive evidence on this point at pres- 

 ent. The programme of the San Diego Station includes an in- 

 vestigation of the vertical distribution and movements of the 

 plankton. It is hoped that precise evidence on this matter of 

 temperature barriers and vertical distribution will be forthcom- 

 ing. The facts are as follows: All species except T. ambulatrix 

 and 2\ longicornis have been found within an area of a few- 

 square miles off San Diego in hauls within 100 fathoms of the 

 surface. A number of hauls from 200-300 and even 400 fathoms 

 have been made, but they do not show any increase in the number 

 of individuals of Triposolenia. The hauls have been made in most 

 cases by lowering the nets to the desired level and towing for 

 10-60 minutes and then bringing the open net to the surface. 

 This affords an assumption in favor of a considerable part of 



