KOFOID : MUTATIONS IN CKRATIUM. 229 



joined without considerable change in their length.^ AVe are therefore 

 left in doubt, and perhaps in total ignorance, of the form of the ancestral 

 skeleton. We can only say with certainty that these two very diverse 

 cells, these two species of Ceratium, have descended from an ancestral 

 cell similar to one of the two or possibly totally different from either. 

 In view of the limited number of cells usually found in Ceratium chains, 

 the sliort time in which chain formation, as a rule, continues and the 

 youth (relative shortness of the apical horns) of both cells, the infer- 

 ence may be made tliat generations which separate them from the 

 ancestral form ai"e few rather than many, and that the relationship 

 between the two cells is near rather tjjau remote. 



In the case of the mutation of C. tripos to C californiense there was 

 evidence in the chain (plates 1-3) that the change was to a slight de- 

 gree graduated, that is, that the second division produced a more pro- 

 nounced type of C. californiense than the first. In the case of C. ostenfeldi 

 and C. californiense the chain is too short to yield any evidence. I have 

 found, however, in the plankton of the Pacific several individuals of 

 C. californiense which exhibit structural features which tend toward but 

 do not attain the characteristics of subgenus Macroceratium. For ex- 

 ample, the following have been recoi'ded : individuals with the lateral 

 curvature of the antapical horns slightly more pronounced than in the 

 normal individual,, and again, with one or both antapical tips squarish 

 (plate 4, fig. 6), but still closed, and one case with one tip plainly open 

 (autotomy ■]). These observations lead me to surmise that the phenome- 

 non of abruptly transitional forma exists also here between the two types 

 involved in this mutation. In the one mutation, and possibly in botli, 

 however, the changes, in com[)arison with the variations of the fluctuat- 

 ing type are: (1) less frequent in occurrence; (2) more extreme in their 

 amplitude ; and (3) involve at the same time a whole coirjplex of funda- 

 mental cliaracters. 



Earlier Observations on Mutations in Protista. 



Mutations in Diatoms. 



A phenomenon similar in some respects to the mutations in Ceratium 



has been found by Miiller (1903, 1904, 1906) in the colonies of Melosira, 



a fresh-water diatom in which the cells are normally found joined together 



in linear filaments or chains. In the same filament Milller found cells 



' The fact tliat in the chain shown in plate 1 two sister cells, the third and the 

 fourtii, have apparently unequal transdianieters, is due to the obliquity of the posi- 

 tion of III3 when sketched. In plate 3 the real dimensions are better shown. 



