86 C. M. Child 
hand, we accept Morgan’s hypothesis that polarity is a gradation 
of substances along the axis, another hypothesis is necessary to 
account for the most conspicuous feature of polarity, viz: the 
fact that the same tissues may produce totally different structures 
according as they constitute one end or the other of the axis. 
In short, neither the hypothesis of directive organization nor 
that of regional gradation alone will account for the characteristic 
phenomena of polarity in animals, for the very simple reason that 
polarity possesses both regional and directive features. In other 
words there are two internal factors in polarity, constitution and 
correlation. In Harenactis or in Planaria the cells-at a given level 
form tentacles or a foot, a head or a tail, according to thecharac- 
ter of their relation with other parts of the organism, 1. e., their 
correlations. ‘This factor of correlation and the part which it 
plays in polarity have been recognized by the botanists, and it is 
certainly no less important in animals than in plants. Whether 
a given cell complex gives rise to an anterior or posterior, an oral 
or aboral structure, cannot possibly depend upon itself alone, since 
it is originally the same in both cases, but nevertheless gives rise 
to different results. 
Morgan’s hypothesis can account only for regional, never for 
polar phenomena. According to his latest statement of the hypoth- 
esis (Morgan ’07, pp. 378 to 380), a head forms at one end of a 
piece because there is more head-forming substance there, and 
a tail at the other end because that contains more tail-forming 
substance. [his amounts exactly to the assertion that heads and 
tails appear where we find them because they do. Moreover, it 
affords us absolutely no basis for understanding how either a head 
or a tail may arise from the same cells according as they form one 
end or the other of the piece. 
On the other hand, Driesch’s hypothesis accounts for these 
differences at the two ends of the axis, but cannot account for the 
phenomena of primary heteromorphosis. Moreover, the evi- 
dence in favor of the hypothesis of a directive organization seems 
at least unsatisfactory. 
I believe that no conceivable hypothesis which concerns con- 
stitution alone, or correlation alone, can account for the phe- 
