Vol. 3] Kofoid. — Species Characters in Triposolenia. 123 



specific types are fitted to survive is witnessed by the coincident 

 occurrence of at least five of the species in the San Diego area, 

 and it is not improbable that prolonged search will bring the 

 other three to light in that region. While it is not probable that 

 all of the species are all equally well adapted to survive it is evi- 

 dent that they have been and are sufficientlxj well adapted to 

 maintain themselves. 



It is conceivable that all variations or mutations which have 

 weakened the coherence of the parts or increased or decreased 

 the specific gravity or specific surface beyond certain limits would 

 result and may have resulted in the extinction of these unfit in- 

 dividuals which have passed the "dead line" into the territory 

 where selection becomes operative. It is equally probable that 

 many types of form and proportions of the structural elements 

 of Triposolenia might occur and may have occurred upon which 

 selection is entirely inoperative by reason of the fact that they 

 lie within the circle of permissable adaptive changes at whose 

 circumference natural selection lies in wait. For example, it is 

 impossible to see how natural selection can account for the tri- 

 angular, elliptical and the two pendant types of midbody in 

 Triposolenia. All afford approximately equal form resistance, 

 specific surface, and exposure for metabolism. That one type is 

 about as effective as another, or at least sufficiently effective, is 

 evidenced by their continuance in a common environment. The 

 same statement might be made with reference to other diagnostic 

 structural features of the several species. In Triposolenia we 

 have an illustration of the occurrence of a number of types of 

 structure, apparently of specific rank, types defined by a number 

 of structural elements rather than by a single one or a few, types, 

 moreover, which are apparently not linked together by coincident 

 variations of the fluctuating nature of all the characters involved, 

 and ihese types, or species as I believe them to be, all appear to 

 lie within the circle of permissible adaptive modifications where 

 natural selection is inoperative. The utility of each of the com- 

 plexes of characters is sufficient for their preservation without 

 the necessity of calling in natural selection to account for their 

 differentiation and continuance. It is not difficult to imagine a 

 number of types of Triposolenia, modifications of the structural 



