40 ■ PROTOPLASM AND THE CELL. 



sound, pressure, etc., and upon this susceptibility depend many of the 

 higher manifestations of life. For instance, waves of light or of sound, 

 acting upon special protoplasmic structures in the eye and ear, call forth 

 actions which ultimately result in the sensations of sight and hearing. 

 Similar considerations apply to the senses of smell, taste, and touch ; but 

 the discussion of all these special modes of protoplasmic action must be 

 deferred. Enough has been said to show that living organisms (that is, 

 the protoplasm which is their essential part) are able to respond to many 

 influences proceeding from the world in which they live. Upon this prop- 

 erty depend the intimate relations between the organism and its environ- 

 ment, and the power of adaptability to the environment which is one of the 

 most marvellous and characteristic properties of living things. 



Non-cliff iisihility. Living protoplasm, like most of the various proteid 

 matters which it yields (p. 36), is indffusihle. It will be seen eventually 

 that osmotic processes play a leading role in the lives of plants and animals, 

 since they are in large part the means by which nutriment is conveyed to 

 the living substance. In view of this fact, the non-diffusibility of proto- 

 plasm as well as of ordinary proteids is a fact of much significance. 



Vegetal and Animal Protoplasm. The protoplasm of plants is es- 

 sentially identical with that of animals in chemical and physical relations, 

 and manifests the same fundamental vital properties. But it would mani- 

 festly be absurd to suppose this identity absolute, for if it were so, plants 

 and animals would also be identical ; and furthermore, the protoplasm 

 of every species of plant and animal must differ more or less from the 

 protoplasm of every other species. What is meant is that the differences 

 between the many kinds of protoplasm are far less important than the 

 fundamental resemblances which underlie them. 



