248 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



2. The instances of its occurrence appear to be rare. This is true in 

 general of the elementary species mutating from Oenothera lamarekiana. 

 With the single exception of an outbreak of Leptinotarsa pallida (in the 

 field) the numbers of heteromorphic individuals obtained in the pedigree 

 studies of Tower (1906) were small. The mutation of Paramecium 

 caudatum to P. aurelia was observed but once by Calkins (1906) in his 

 many cultures of this protozoan. 



It is obviously no necessary criterion of a mutant that it should be 

 rare. It is conceivable, indeed quite probable, that under certain suitable 

 conditions in nature mutations might be called forth in great numbers 

 at one time. The fact is merely stated that mutations have been seen in 

 but relatively few instances among many of normal reproduction observed 

 in Ceratium, and that this coincides with observations on mutation else- 

 where. In this connection it is well to note that in view of the relative 

 simplicity of their structural features, rapidity of reproductive processes, 

 and the ease under favorable conditions with which great numbers of 

 individuals can be assembled and examined, the Protozoa present unique 

 opportunities for attack upon the problem of mutation. It is possible, 

 for example, in a few days to pass in critical review more Ceratium than 

 fishes, birds, or mammals in a lifetime. 



3. The instances of its occurrence suggest the action of environmental 

 factors in producing the mutations. In one at least of Lohmann's 

 instances and in one of my own the parent cell had undergone autotomy. 

 The distribution of autotomized Ceratium in the plankton of the sea 

 suggests, as I have elsewhere (1908) shown, the action of environmental 

 changes, possibly those resulting from the sinking of the organism or the 

 vertical circulation of the water. Autotomized individuals of G. tripos 

 balticum (forma tnmcata Lohmann) were rather common in the Baltic 

 plankton at the time when the heteromorphic chains were found by 

 Lohmann in aquaria stocked from the Baltic. As I have elsewhere 

 noted, Lohmann's seasonal tables of physical data indicate a consider- 

 able change in the Baltic in temperature and salinity, suggestive of an 

 unusual amount of circulation prior to and during the time when the 

 heteromorphic chains appeared. Chains, however, were found only in 

 aquaria where naturally the conditions as to illumination, movement 

 of the water, and extent and character of temperature changes differ 

 from those in the sea. It is possible that some of these changes were 

 potent in stimulating Ceratium to mutation here. 



The two cases of mutation found by me occurred about 1500 miles 

 apart in the eastern tropical Pacific, but both in localities with certain 



