Studies on Regulation. IV. 131 



curately the continually changing movements of these animals, but 

 I think no one who actually observes the pieces can fail to be 

 convinced of their importance in determining form. It is true of 

 Leptoplana as of Stenostoma (Child, '02) that it has no "normal 

 form," t. e., no definite hard and fast form inherited and devel- 

 oping In the Individual Independent of physical conditions. What 

 these animals do possess Is a capacity for certain kinds of activity. 

 These are given potentially in the chemical and physical structure 

 of the protoplasm, which to my mind represents rather capacity 

 for functional activity in the broadest sense than form. As my 

 experiments prove, certain elements of form In the morphological 

 sense develop incidentally as the result of functional activity In 

 in a given environment. These elements have been commonly 

 regarded as typical and determined by heredity because they are 

 common within certain limits of variation to all individuals of 

 a species, but when we consider that under natural conditions 

 both functional activity and environment are essentially similar 

 in different individuals of the species the reason for likeness In 

 these form-elements becomes clear. It is only when we can alter 

 the functional activity as I have done experimentally in the case 

 of Leptoplana, or the environment, as I succeeded in doing for 

 Stenostoma (Child, '03a) that the dependence of these elements 

 of form upon these two factors becomes clearly evident. 



But these experiments concern only morphological characters 

 of a certain kind. Experiments of others have already shown that 

 "formative factors" are many and various, and generalizations 

 from the consideration of a single group of characters are unsafe. 

 It win never be possible to explain form on the basis of a single 

 principle. All the complex activities of which organisms are 

 capable are "formative factors" : when we can view all of these 

 In their complex interrelations and know the part which each 

 plays, then and only then shall we "understand" organic form. 



The relation between form and heredity has never been satis- 

 factorily determined. With the advance in our knowledge the 

 fact becomes more and more evident that the organism is not 

 merely a complex of structural elements ready made by heredity 

 for certain functional activities, but rather a complex of ac- 



