282 THE AMERICAN NATURALIST [VOL. XLIII 



ular variation before the latter can be regarded as giving 

 evidence of a corresponding determinant. The burden 

 of proof is on the shoulders of the upholders of the doc- 

 trine of determinants and it is a far heavier one than the 

 champions of this doctrine commonly appreciate. Let us 

 suppose that among the various sets of hereditary tend- 

 encies that find expression in the organization of an indi- 

 vidual one should include the production of a particular 

 variation in a single small part. The constitutional differ- 

 ences which may go along with this peculiarity and of 

 which it may be regarded as one expression may be modi- 

 fied or kept from becoming manifest by other and rival 

 sets of hereditary tendencies thereby rendering it almost 

 impossible to detect the correlations that really exist, and 

 giving the character the delusive appearance of independ- 

 ence. The question of the independent variability of 

 parts is a crucial one for the particulate theories of in- 

 heritance, but it is one. so beset with practical difficulties 

 that a final answer may not soon be forthcoming. 



In the preceding discussion the attempt has been made 

 to show that the various categories of variations recog- 

 nized by De Vries and others are not sharply separable 

 either on morphological grounds or by their behavior 

 when subjected to crossing experiments. The attempt 

 was made also to show that neither the facts of variability 

 nor those of Mendelian inheritance give any support to 

 the doctrine of pangens, determinants, or other assumed 

 bearers of unit characters, and that unit characters, as 

 elements than can enter or depart from the complex of 

 tendencies that make up an organism probably have no 

 existence. It is evident that variations differ in their 

 stability, but the explanation of this fact may lie in the 

 physiological relations of the variation rather than in 

 some hypothetical representative unit. Whether the 

 variations of the discontinuous type have been influential, 

 in any marked degree, in shaping the course of evolution 

 is a question upon which we need much more evidence. 

 Mutations, as we have seen, may be very small affairs. 



