TROPHIC ACTION. 33 



necessary to growth, therefore growth is a sign of 

 nutrition. Rapidly growing tissue is believed to 

 be highly nourished, because it is found to be very 

 vascular; and often we may infer the degree of 

 nutrition from the degree of vascularity. 



That kind of nutrition or vital action which pro- 

 duces growth, or change in the material structure of 

 the subject is conveniently termed " trophic action ; " 

 we may speak of expression by " trophic action," 

 and compare it with expression by movement. 

 When trophic action is the mode of expression, we 

 find some histological change in the subject of 

 growth ; when a movement is the mode of ex- 

 pression, there is often no change in the structure of 

 the subject that can be observed as a histological 

 fact, and if we neglect to observe and record that 

 movement, we possibly neglect the only objective 

 sign of the change occurring in the subject. Nutri- 

 tion and growth, when they occur together, indicate 

 or express life in the subject; it follows, then, that 

 any of the expressions of nutrition and growth are 

 expressions of life. 



Permanent impressionability is a very important 

 property, and it was shown in the second chapter 

 that retentiveness may alike be found in animate 

 and in inanimate subjects. Retentiveness in living 

 things may be indicated or expressed in various 

 ways. When in a subject a certain stimulus is 

 always followed by the same expression, we may 

 conclude that the subject -is unchanged, or reten- 

 tive at least of the property thus expressed. 

 Retentiveness is, then, not a sign of evolution or 



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