HEREDITY FROM PHYSICO-CHEMICAL POINT OF VIEW. 87 



joining region. A similar reciprocality of influence is especially 

 characteristic of physiological processes, such as excitation and 

 inhibition, e. g., in the central nervous system; and it is also well 

 known to be characteristic of various processes of growth and 

 regeneration in both animals and plants. 1 This is why (for 

 example) cutting off a tubularian head enables an adjoining 

 region of the stem to form a new hydranth ; the region where the 

 new growth takes place has been removed (by the operation) 

 from the inhibiting influence of the original hydranth. Similarly 

 a short piece of iron wire which is in contact with a piece of zinc 

 will not form filaments in ferricyanide solution until the zinc is 

 detached or otherwise rendered inactive. Cutting away the 

 zinc thus initiates the development of filaments from the iron; 2 

 the structure-forming process had previously been inhibited by 

 the activity at the zinc, which on account of its greater tendency 

 to send ions into solution alone forms filaments while the two 

 metals are in contact. To express the matter biologically: the 

 zinc seems actively to appropriate the available structure- 

 forming material (ferricyanide), and in so doing prevents the iron 

 from utilizing this material to form filaments. Similarly the 

 hydranth, with its higher rate of metabolism, acts as the chief 

 structure-forming region in the tubularian, and inhibits structure- 

 formation of the same kind in its vicinity. 3 It corresponds, in 

 this sense, to the anodal metal in the local electrical couple of 

 zinc and iron. The growth-initiating consequences following 

 physiological isolation to use Child's concise and illuminating 

 expression 4 may thus be instructively simulated by means of 

 an inorganic model of this kind. 



These and similar parallels appear to indicate that the same 

 type of process is concerned in the structure-formation in the two 

 kinds of system, otherwise so entirely unlike in character. 5 If 



1 Loc. cit., pp. 156, 163. 



2 Loc. cit., pp. 152 seq. 



3 This is an example of the dominance or control of formative processes by those 

 regions having the highest rate of metabolism: cf. Child's "Senescence and Re- 

 juvenescence," Chapter 9, p. 210; also "Individuality in Organisms," Chapter 5. 



4 Loc. cit. 



6 It is a question whether an electric current passing between any semi-permeable 

 water-insoluble phase and the adjoining aqueous solution can do so otherwise 

 than by a process of ionization (or deionization) at the boundary, i. e., by a process 



