380 



ORGANIC GROWTH 



SEC. 



Growth, therefore, does not necessarily produce visible 

 alterations — the changes which precede an increase in size 

 are growth, and so are alterations in the position of particles 

 (molecules) which produce no increase in size ; such a change 

 of position may only cause an alteration in the form of the 

 body, but it may even produce a diminution in size. 



Growth is by no means necessarily the result of the 

 assimilation of food : the action of any external stimuli is 

 capable of causing changes in the position of the particles 

 of the body, and thereby of causing growth in my sense of 

 the term. The idea includes every inheritable alteration 

 either of material or form in the organism, or (as underlying 

 these) every alteration of the interaction of forces in the 

 organism ; and therefore the result produced by stimuli 

 influencing the protoplasm indirectly is growth. Therefore, 

 also, even diminution in size and degeneration is grow^th. 

 Thus plants and animals grow to a smaller size in the north 

 and on mountains than in the south and in valleys, and the 

 exclusive influence of one condition, abundance of food, leads 

 to the degenerate processes of growth which parasitic forms 

 exhibit in organs related to other conditions. 



Thus two things are necessary to produce growth (1) the 

 given composition of the organism ; (2) the action of stimuli 

 (food being considered as a stimulus). It was in reference to 

 the former that I described in previous works some of the 

 causes of the modification of forms as " internal " or " consti- 

 tutional causes." 



Since living Ijeings differ from one another in the composi- 

 tion of their bodies, so stimuli act differently upon them, 

 produce in some changes different from those they produce in 

 others : they grow in different w^ays. 



The constitution of the body, which has so important an 

 influence in determining the course of growth in an organism, 

 is to a very great extent the result of the inheritance of 



