chap, i.] CHARACTERS OF LIVING MATTER. 5 



some apparent friend or enemy in the water, or by a 

 touch from our needle, as we observe it under the 

 microscope, a mass of living matter will be found to 

 be irritable. In consequence of this irritability 



it undergoes some change converting latent into 

 actual energy, and this is most frequently and most 

 easily seen to be some change in space, or in the 

 relations of its parts ; these are due to what is known 

 as the contractility of living matter. In other 

 cases, the production of heat, light, or electricity, is 

 the expression of irritability. 



We have next to observe, that within the area of 

 any given mass of protoplasm, there may be move- 

 ments of its parts ; some of the granules seem to 

 stream in a- more or less regular course between 

 those on either side of them, in a way which can best 

 be understood by supposing the observer to be raised 

 above and to be able to note the movements of a great 

 crowd of passengers in a busy street ; some move 

 faster than and overtake others, some collect into 

 more or less small crowds ; others, having moved on- 

 ward for a certain distance, turn aside or turn back. 

 This streaming movement of protoplasm is highly 

 characteristic, and affords a proof that the problem of 

 the motile activity of protoplasm can only be explained 

 by the study of the parts of which it is made up. 



Lastly, thin layers of non-granular protoplasm 

 are sometimes to be observed gliding' over firm 

 bodies ; by these means the whole mass is enabled to 

 progress in a forward direction. 



The study of streaming movements shov/s us that 

 the constituent particles do not move around any fixed 

 point, but freely as the particles of a fluid substance. 

 So far as we can see, these movements are not the 

 result of any external cause ; did we choose to allow 

 that a simple mass of protoplasm had a " will," we 

 might well call them " spontaneous" or " voluntary;" 



