CAUSAL RELATIONSHIPS OF GROWTH AND DEVELOPMENT 23 



they only indirectly govern whatever changes they initiate, it follows 

 that the actual energy of an applied stimulus need neither be great, 

 nor bear any direct relation to the magnitude of the changes induced 

 by its application. A supply of energy must have been previously accu- 

 mulated in the responding organ, and the stimulus acts upon this stored 

 energy like a match applied to a barrel of gunpowder. Nor is it necessarily 

 an essential preliminary to a response that the responding organ should be 

 in a labile condition, or be in unstable equilibrium l . 



Since it is the living organism which forms the subject of all physio- 

 logical study, the meaning, importance, and action of the factors which 

 constitute the external world may be most conveniently studied in con- 

 nexion with the properties and functions of the organism on which they 

 are brought to bear. That is to say, it is wholly incorrect in principle 

 to commence with the external environment, and describe in succession 

 the different physiological reactions produced by each external agency. 

 Probably no external agency is absolutely inactive upon plants, and indeed, 

 with the sole exception of magnetism, every form of energy with which 

 we are acquainted exercises some physiological action or other. 



External agencies either act as stimuli producing disproportionate 

 results, or else they originate interactions in which the transfer of energy 

 from agent to organism takes place in definite ratio and equivalent amounts. 

 But it is also possible to regard the external factors and the results they 

 produce from other points of view ; for example, from that of the purpose 

 aimed at, the value of the reaction to the plant, the form which the 

 reaction assumes, or the way in which stimulus, reaction, and final result 

 are linked with one another. Since, however, none of these methods 

 introduces any new principles, it is needless to discuss them further. 



SECTION 4. The Causal Relationships of Growth and Development. 



Hitherto we have been considering isolated functions, but the same 

 general principles which govern these apply also to the functional complex 

 which constitutes an organism, in every stage of its development, and hence 

 are applicable to all the processes involved in the successive stages of its 

 ontogenetic progress. The process of development is, indeed, extremely 

 complex, and involves such a linking together of causes and effects, that 

 automatic changes of disposition and correspondingly altered activities 

 are being continually produced and propagated. 



That some such causal relationship does actually exist is a necessary 

 postulate, although our present knowledge is insufficient to enable us to 

 directly refer the progress of development, which results in the production 



1 Pfeffer, Die Reizbarkeit, &c., 1893, p. 14. 



