No. 542] DARWIN'S THEORY OF EVOLUTION 79 



ontogenic modifications are, for the most part, clearly 

 distinguished as belonging to another category. 



Similarly Lutz 5 has kindly analyzed for the writer 

 twenty-one cases cited by Darwin among insects as possi- 

 ble material for selection and as indicating modifications 

 by environment. Eight of these cases are doubtful; 

 seven cases represent ' ' gradations,'' or "slow degrees," 

 probable or possible continuity; six cases represent dis- 

 continuity. It seems to Dr. Lutz that Darwin made the 

 most of the cases of discontinuous variation he knew 

 about, but that for the most part he had continuous 

 variation in mind. 



It is therefore of interest, in view of the neglectful and 

 almost contemptuous attitude of certain writers toward 

 Darwin's observations, to make a fresh summary of the 

 principles found scattered through the pages of his great 

 work on "Variation," reexpressing some of these prin- 

 ciples in modern terms. 



Darwin on ' ' New Characters ' ' and ' ' Individual Differ- 

 ences" 6 as Expressed in Modern Terms 

 (1) Newly appearing characters arise from unknown 

 causes, either stable in heredity, variable in heredity, or 

 not hereditary at all. 7 (2) "Individual variations" are 

 minor suddenly appearing characters, heritable. (3) 

 Characters of all kinds, whether new or old, tend to be 

 inherited. Those which have already withstood environ- 

 ment will, as a rule, continue to withstand it and be truly 

 transmitted. (4) A change of environment is principally 

 but not invariably the source of new variations. (5) 

 There are periods of variability or mutability in which 

 many new characters appear. (6) New characters are 

 observed to accumulate in successive generations in the 



•Frank E. Lutz, letter to the writer, December 15, 1911, "References to 

 insects in the 'Origin of Species/ especially those bearing on the question 



