PROTOPLASMIC FUNCTION 



43 



FIG. 17 



stimulus from one part of an animal to another that a special tissue, 

 nerve, has evolved so that it may be done more promptly and more 

 accurately. The nervous system, then, aside from the important sup- 

 posed relation to spontaneity and consciousness, is only an immensely 

 complicated series of protoplasmic paths through the various parts, 

 large and small, of the body, and connecting by means of the sense organs 

 the individual animal with its environment. This main function of 

 conduction is discussed in the chapter on the Nervous System. 



TAXES. This term is used to indicate several sorts of reactions to the 

 stimulation of protoplasm. It has largely replaced the older word 

 tropism. The term is from the Greek taxeo, to arrange, and in phy- 

 siology indicates a tendency which various animals and plants exhibit of 

 arranging or coordinating themselves in various ways to different sorts 

 of stimuli or conditions in their 

 environments. Especially does 

 the word taxis indicate the attrac- 

 tion toward the places where 

 these conditions or stimuli are, 

 or away from them. The most 

 important of the taxes, only re- 

 cently investigated, and then with 

 not very important results, are 

 chemotaxis, attraction toward 

 certain substances; thermotaxis, 

 toward heat; phototaxis, toward 

 light; electrotaxis, toward elec- 

 tricity; and barotaxis, toward 

 pressure, including thigmotaxis, 

 rheotaxis, and geotaxis. Though 

 they are interesting, and possibly 

 of some importance in explaining the behavior of animals, space does 

 not allow a further discussion of these reactions in this place. 



CONSCIOUSNESS has such a close relation to the living substance that 

 it may be mentioned here, for convenience, without in any sense giving 

 it classification as a manifestation of the irritability of protoplasm. 

 Whatever consciousness or mental activity may be (and it cannot be 

 defined except as experience), it has a relation to protoplasm such that, 

 so far as we actually know, it does not exist apart from bioplasm. No one 

 at present considers it a function of protoplasm or a product of proto- 

 plasmic life. Thought is no longer said to be "the secretion of the brain, 

 as urine is of the kidney," but it is in some quite unknown way related 

 to the irritability of protoplasm, and is at once master and servant of the 

 living substance of animals and an accompaniment of its life. The 

 chief usefulness of consciousness (if for the sake of system we must find 

 a biological "function" for it) is probably resident chiefly in memory. 

 It is the means by which experiences are acquired for preservation in 

 memory to be of further use to the individual. (See Chapter XII.) 



A diagrammatic section through the photogenic 

 organ of a lightning-bug (Pyrophorus noctilucus) : 

 C, cuticle; h, hypodermis; O, luminous organ; S, 

 blood-sinus of the organ; a, adipose body; tr, 

 trachea?; m, muscles; n, nerves. (Dubois.) 



