220 bulletin: museum of comparative zoology. 



C. (Biceratiuni) californiense is found in chain with C. (Macroceratium) 

 ostenfeldi. Both the amplitude and the direction taken in these abrupt 

 changes in form are of great interest, and are, I beheve, of profound sig- 

 nificance in their bearing upon the question of the method of organic 

 evolution in natm-e. 



The species in or between which these mutations occur are well- 

 established species of the genus, have a wide distribution in the sea, and 

 in the main, excluding questions of synonymy, a general recognition in 

 the literature of the subject. The evidence of this abrupt change in 

 form, this mutation, or perhaps we may even say transmutation, of 

 species seems unequivocal. The facts upon which these conclusions rest 

 are as follows. 



The Mutation of Ceratium tripos to C. californiense. 



The chain exhibiting this change (plates 1-3) was taken at 8 A. M. 

 Jan. 24, 1905, at Sta. 4737, 19° 57' 30" S., 127° 20' 18" W., on our 

 line between the Galapagos Islands and Manga Reva in an intermediate 

 haul from 300 fathoms to the surface. It consists of four individuals in 

 a chain which has resulted from two succeeding cell divisions. The four 

 individuals are in the third generation, that is, are granddaughter cells 

 of an original grandparent cell. The original skeleton of the grandpar- 

 ent cell is now widely severed, its anterior moiety forming the forward 

 armor of the foremost (uppermost in the plates) individual, and its 

 posterior moiety the rearward shell of the hindmost (lowermost in the 

 plates) in the chain. 



Chronology and Nomenclature of Skeletal Parts of Chain. 



An analysis of the chronology of skeletal formation and the distribu- 

 tion in the present chain of the skeletal parts formed in the three gen- 

 erations is illustrated in the accompanying diagram in which Aj A^ A3 

 and Pi P2 Ps represent the anterior and posterior moieties of the skele- 

 tons formed on the first, second, and third generations respectively. 

 For convenience in referring to the individuals in the following dis- 

 cussion they are numbered from the anterior end posteriorly, with the 

 distinguishing number of the generation added as subscript ; thus the in- 

 dividual I3 is the first (foremost) individual in the third generation, and 



its skeleton ^^ consists of the anterior moiety of the grandparent cell of 



the chain and a posterior moiety formed at the latest division. It is 



