DYNAMICS^ — MORPHOGENESIS AND INHERITANCE 431 



whether produced by low temperature or other external physical 

 or chemical agents or by physiological conditions, will deter- 

 mine complete absence of eyes and of most or all of the ganglionic 

 mass. The series of heads from normal to acephalic is physio- 

 logically a series whose terms differ primarily in a quantitative 

 rather than a qualitative way, the qualitative differences which 

 arise resulting from the quantitative differences. The same is 

 true of any other graded series of forms resulting from differential 

 susceptibility; e.g., the sea-urchin modifications (Child, '16 b). 

 Planaria does not of course inherit any particular type of head, 

 but merely the potentiality of giving rise under conditions differ- 

 ing in degree or quantity to a graded series of head-forms, with 

 an indefinite number of terms in the series. The hereditary fac- 

 tors, genes, or whatever we prefer to call them which are con- 

 cerned in head formation in Planaria, are unquestionably all pre- 

 sent in all forms of head but the sort of head which appears is 

 determined on the one hand by quantitative physiological differ- 

 ences between different regions of the body and on the other 

 by primarily quantitative alteration of these conditions through 

 the non-specific action of physical or chemical factors. 



The physiological gradients of the whole body of Planaria or of 

 any other axiate form are primarily quantitative, physiological 

 feature, very similar in character in specifically different proto- 

 plasms, and such gradients are merely factors in determining the 

 first steps of the process of realization of hereditary potentiali- 

 ties. A physiological gradient is merely a quantitative physio- 

 logical factor affecting the action of the hereditary mechanism 

 of a specific protoplasm. As pointed out elsewhere, it is the 

 primary pattern of the organism (Child, '20 b). The protoplasm 

 of the species with its hereditary constitution representing a 

 wide range of potentialities is the material in which the pattern 

 is, so to speak, worked out. The pattern, in its simplest terms 

 the physiological gradient, is the organizing factor. The gradient 

 determines localization, order, proportion, and even presence or 

 absence of particular parts, but the specific characteristics of 

 the parts present are determined by the hereditary constitution 

 of the protoplasm. 



