430 C. M. CHILD 



When the conditions are altered, the actual behavior of the 

 protoplasmic system is altered and other potentialities than the 

 normal are realized. 



As its title indicates, this series of papers has been regarded 

 as a study of inheritance, as well as of development, because the 

 facts of development established afford a basis for certain con- 

 clusions concerning inheritance. As regards the head of Planaria, 

 for example, it is evident that all the different forms of head 

 are just as truly inherited as the normal; that is, all of them 

 represent potentialities of the physicochemical system which is 

 the material substratum of inheritance. Which of these potenti- 

 alities is realized depends upon the conditions of development. 

 Or we may say if we wish that under certain external conditions 

 of development a particular head-frequency is inherited for 

 a certain size of piece from a certain body-region of animals 

 of a certain size of physiological age and a certain nutritive con- 

 dition. This, however, means exactly the same thing as the 

 preceding form of statement. 



If we attempt to retain a strictly preformistic conception of 

 inheritance, the facts in the case of Planaria force us to the 

 conclusion that a grouping or association of determinants, genes, 

 or factors must exist for each form of head occurring in develop- 

 ment, a particular grouping being activated by each particular 

 complex of developmental conditions. But the head-forms 

 constitute a graded series differing primarily in degree and an in- 

 definite number of forms is possible. Any preformistic view 

 which regards these groupings as actually existing in space and 

 time in the germ plasm rather than as mere possibilities of action 

 in a complex physicochemical system, leads either to absurdity or 

 to some form of VitaUsm.' 



The point to which I wish to call particular attention, however, 

 is that developmental conditions which differ primarily in a 

 quantitative rather than in a qualitative way may determine 

 qualitative differences in the developmental realization of the 

 hereditary potentialities. In other words, they may determine 

 presence or absence of a particular organ or character. A cer- 

 tain degree of retardation or inhibition of development in Planaria 



