No. 509] THE CATEGORIES OF VARIATION 281 



parts, is just what renders the proof of this theory ex- 

 ceedingly difficult. The alleged independent variability 

 of parts is Weismann's strongest proof of his doctrine 

 of determinants. If a pit in the e.ar or a white tuft of 

 hair on the head can be transmitted for several genera- 

 tions without involving any other change in the organism, 

 we are forced to assume, according to Weismann, that 

 there is a small part of the germ plasm varying independ- 

 ently of the rest which forms the basis or determinant 

 of this character. But the contention particularly diffi- 

 cult of proof is that the characters really do appear in 

 independence of the other parts of the organism. A 

 variation may conceivably depend upon a general change 

 in the constitution of the substance of heredity, although 

 manifesting itself most conspicuously in a single part. 

 A pit in the ear may be the most obvious sign of the very 

 slight constitutional differences between two individuals. 

 It is common to find two closely allied species or varieties 

 differing markedly in one or two features and much less 

 conspicuously in numerous other parts of their structure. 

 Peculiarities of horns are sometimes associated with less 

 noticeable characteristics of the hair, thus pointing to a 

 common origin of these features in some general modifica- 

 tion of the ectoderm which in turn may result from some 

 change affecting the germ plasm as a whole. Albinism 

 which is so often cited as a unit character is a peculiarity 

 of far reaching correlations, being often associated with 

 impaired sight or hearing, diminished fertility, and even 

 lessened power of resistance to disease. 



To establish the independent variability of parts re- 

 quires a much closer study of possible correlatiojns than 

 has yet been made. The task is rendered particularly 

 difficult by the varied combination and segregation of 

 ancestral tendencies, which we have just considered. If 

 we can account for the independence of certain characters 

 on the ground of combining and sorting ancestral tend- 

 encies as wholes, one has to disprove the possibility of 

 applying this explanation of the appearance of a partic- 



