Vol. 3] Kofoid. — Species Characters in Triposolenia. 121 



II. The Adaptive Significance of Specific Characters. 



In view of the current discussion among biologists of the 

 adaptive significance of diagnostic characters which separate spe- 

 cies and of the efficiency of natural selection in the origin of these 

 characters, individually or in their totality, it is a matter of in- 

 terest to consider what light the organisms with which we are 

 here dealing throw upon this vexed question. The relative sim- 

 plicity of their organization and the fact that the specific distinc- 

 tions are comparatively few, probably less than a dozen being 

 revealed in the theca on the closest scrutiny, and further that 

 they are expressed in the more permanent exoskeleton or thecaP 

 wall where they are preserved in fixed form for comparison, and 

 of still greater significance the fact that they have had their 

 origin in the surface waters of the sea, a region of relative en- 

 vironmental uniformity and possibly the primitive home of or- 

 ganisms (Brooks '95), all conspire to reduce the problem to its 

 lowest terms and to lend an unusual interest to its consideration. 



The problem is here freed from the complexities which attend 

 its consideration in the higher organisms where multiplicity of 

 parts makes difficult the expression of the total specific differ- 

 ences, where the mobility and contraction of the organism or its 

 constituent parts renders impossible a precise definition of all 

 the diagnostic characters, and where the period of growth is long 

 continued and the total form-cycle of the individual in which the 

 species characters are expressed is more extended in time and of 

 greater amplitude in its modifications. Incomplete as our con- 

 ception of the ensemble of specific characters in Triposolenia 

 uuist be, the conditions are such as to afford a nearer approach 



' It is of course probable that all of these skeletal characters exhibited in 

 the non-living cellulose skeleton are the expression of the many coordinated 

 internal metabolic processes of the nucleus and cytoplasm of the cell, pro- 

 cesses which, moreover, are of an ephemeral nature in the cyclic process 

 which culminates in the new formation of the half -theca at the time of cell- 

 division when the j^arental theca is parted in the sagittal plane and each 

 daughter cell reproduces the lacking half of the thecal wall. Sexual repro- 

 duction is as yet unknown in this family and, indeed, has been rarely ob- 

 served in the Dinoflagellates as a whole, and is as yet unrecorded among 

 the marine species. Judging from the trend of discoveries in the life- 

 histories of the Protozoa its occurrence is to be expected generally among 

 the Dinoflagellates. Should it occur it is probable, from the observations of 

 Zederbauer (:04), that complete formation of a new theca about the zygote 

 or its offspring will be found to take place after union of the gametes. 



