CHANGE OF SHAPE IN PLANARIA. 289 



machine by human agency. When the machine is constructed it 

 begins to function. 



This point of view seems to me to be essentially naive and 

 anthropomorphic : moreover it is responsible for certain " Schein- 

 probleme " which have arisen from time to time, and for certain 

 barren lines of research, particularly in zoology, which has been 

 dominated by hypotheses of this general character to a much 

 greater extent than botany. 



To my mind life is inconceivable without some sort of function 

 in some sort of structure ; a living structure functions as long as 

 it is alive ; its function is its life. If this view is correct, the 

 organism is functioning in some manner at all stages of its exist- 

 ence, from the earliest to the latest. Function in this sense is 

 physiological activity of all kinds, all transformation and transfer- 

 ence of energy. 



It is a well established fact that the special functional activity 

 of organs is very generally a factor in their development and 

 differentiation beyond a certain stage, and in the maintenance of 

 their characteristic form and structure after development is com- 

 pleted. In the absence of this functional activity the organ does 

 not develop beyond a certain point and does not persist indefi- 

 nitely. Functional hypertrophy, atrophy from disuse, functional 

 adaptation, etc., are terms used to designate these relations between 

 function and form. 



But the real difference between earlier and later stages of 

 development consists, it seems to me, not in the absence of func- 

 tion at one stage and its presence at another, but rather in the 

 difference in character of function in the different stages. If a 

 particular kind of function determines the structure and differentia- 

 tion at one stage, and we know that it does this, is there not a 

 logical basis for the belief that the functions which exist in other 

 stages are also formative factors in those stages ? 



In other words, is there not good reason to believe that every 

 stage of development is directly or indirectly the necessary con- 

 sequence of the functional activity in the preceding stage ? Ac- 

 cording to this view each stage of development is a machine in 

 the broad sense and each is the product of the activity of a pre- 

 existing machine. External factors play a part, particularly in 



