CHAPTER XIII 



INTERMEDIATES BETWEEN VARIETIES AND THE 

 "PURE LINES" OF JOHANNSEN. 



MISUNDERSTANDING in regard to the physiological sig- 

 nificance of intermediates has been a fertile source of error, 

 and more than any other perhaps has tended to obscure the 

 true interpretation of genetic phenomena. The existence 

 of intermediates has been often alleged as a proof that 

 segregation does not occur in regard to the particular 

 variations towards which the gradational forms seem to 

 lead, and the misuse of statistical method so frequent in 

 biometrical attempts to investigate heredity has come about 

 chiefly through misinterpretation of the nature of such 

 gradational forms. 



These errors are of course a legacy from the period of 

 the essayists, when evolution at large was the chief object 

 of study, and an examination of variation in detail as 

 occurring in specific instances had not been undertaken. 



Biologists committed to the proposition that varieties 

 arise through the transformation of masses of individuals 

 by the selective accumulation of minute differences saw that 

 with each new case in which discontinuous variation could 

 be proved to occur, the scope for their views was reduced, 

 and the existence of intermediates constituted the most 

 promising line of defence. When intermediate and gra- 

 dational forms could be found, the contention might always 

 be hazarded that the definite and extreme forms had been 

 reached by transition through them. 



Now the application of analytical methods to those 



