122 University of California Publications. [Zoology 



to a comprehension of the total organism than can be attained in 

 a more complex organism living in a more diversified environ- 

 ment and descended through a more diversified ancestry. 



Are the diagnostic characters of the species of Triposolenia 

 adaptive ? Do they have a survival value to the organisms which 

 possess them? Does the survival value attach itself to the sum 

 total of the species characters or to but one or several, and are 

 the remaining ones in this case to be "explained" as a result of 

 coordination ? 



If the differentiation of the species in this genus is the result 

 of a struggle for existence it necessarily follows that a survival 

 value attaches to the differential characteristics, either to their 

 sum total, which in the case of Triposolenia we find to include a 

 considerable number of correlated elements, or to one or more 

 of the individual structural characters to which all the others 

 nnist then be linked in some necessary relation that is expressed 

 by the term coordination. In assigning natural selection as the 

 cause of the species characters in Triposolenia we are at once 

 confronted by the difficulty of finding any evidence of the differ- 

 ential survival value of any of the characters in question. Pre- 

 sumably competition is severest between individuals of the same 

 or closely related species and species of related genera. It is, 

 however, impossible to establish the fact of any advantage accru- 

 ing to one of these species over its nearest allies by reason of its 

 structural distinctions and difficult to find any satisfactory basis 

 for a logical inference or conclusion to that effect. 



The structural adaptations in Triposolenia are evidently di- 

 rected toward the development of a specific skeletal surface 

 sufficient for the necessities of flotation, the distribution of skele- 

 tal matter with reference to orientation, and to the exposure of 

 the protoplasmic contents to the action of light and of substances 

 in solution in the water. It is conceivable, and indeed probable, 

 that all the types of midbody, of form, proportions and curva- 

 tures of the antapicals and neck are adaptive, or at least are not 

 unadaptive in these directions, and that the pitted surface of 

 T. truncata is adaptive in compensation for its shorter horns, but 

 the various forms of head and antapical tips and the distribu- 

 tions of tubercles are less clearly of utility. That all of these 



