144 CARL DOWNEY LA RUE 



characters also vary with different stages of maturity of the individuals 

 and it is admitted by JENNINGS himself that one cannot be certain that 

 only mature specimens are being measured. 



Turning now from reinterpretation of old work to the newer experimen- 

 tal data, we find that several studies have been interpreted as showing 

 positive selection effects. Thus STOCKING (1915) found that it was pos- 

 sible to produce distinct groups by selection within certain lines of abnor- 

 mal Paramecia. MIDDLETON (1915) was able to produce strains of 

 Stylonychia, distinct in regard to rate of fission, and this almost at will. 

 JENNINGS (1916), with much greater difficulty, detected small changes 

 in DifHugia, which he attributed to selection. ROOT (1918) and HEGNER 

 (1919), working with Centropyxis and Arcella, respectively, secured data 

 in conformity with those of JENNINGS with DifHugia. 



To this later work, however, certain objections have been made. STOCK- 

 ING'S results were obtained with abnormal forms and it seems to have been 

 generally concluded that normal variations might not necessarily behave in 

 the same way. MAST (1917) secured two groups distinct for rate of 

 fission in Didynium nasutum, without selection, presumably by a mutation 

 in one group. This has some bearing on MIDDLETON'S results, and if it 

 be argued that very frequent mutations would be needed to account for 

 the rapid changes produced by MIDDLETON'S selections, it may be pointed 

 out that great instability must obtain in an organism to allow of such 

 changes even by selection. 



MORGAN (1916) suggests that the selection results with DifHugia may 

 be due to random distribution of discrete particles of nuclear material 

 (chromidia) to the pairs of offspring. If such were the case, and if the 

 bearers of particular hereditary characters were contained in the chromi- 

 dia, one would not expect the effects of selection to be indefinitely cumula- 

 tive. Moreover, the variations would necessarily be of a discontinuous 

 type, and perhaps remotely comparable to non-disjunctional mutations 

 or somatic mutations. The same suggestion would also apply to the 

 res'ults of selection in Centropyxis and Arcella. In all three cases it is 

 likely that the variations dealt with are not of the usual continuous type. 



LASHLEY (1915, 1916) has produced evidence that PEARSON'S criticism 

 of JOHANNSEN'S work is not well founded, and that the parent-offspring 

 correlation is not reliable as a criterion in selection work. 



It must be obvious that the question as to whether variations occurring 

 within a pure line are purely somatic and non-heritable or not, is one that 

 is far from a definite answer. Further investigations on forms suitable 

 for the study of the problem, wherever they may occur, are greatly needed. 



