INTR OD UCTION. 29 



as it is to define contractility as a property of muscle-tissue. But the short- 

 ening of a muscle is a physical phenomenon that can be observed with the 

 senses be measured and theoretically explained in terms of the known prop- 

 erties of matter. Psychical states are, however, removed from such methods of 

 study; they are subjective, and cannot be measured or weighed or otherwise esti- 

 mated with sufficient accuracy and completeness in terms of our units of energy 

 or matter. There must be a causative connection between the objective changes 

 in the brain-cells and the corresponding states of consciousness, but the nature 

 of this connection remains hidden from us ; and so hopeless does the problem 

 seem that some of our profoundest thinkers have not hesitated to assert that it 

 can never be solved. Whether or not consciousness is possessed by all animals 

 it is impossible to say. In ourselves we know that it exists, and we have 

 convincing evidence, from their actions, that it is possessed by many of the 

 higher animals. But as we descend in the scale of animal forms the evidence 

 becomes less impressive. It is true that even the simplest forms of animal 

 life exhibit reactions of an apparently purposeful character which some have 

 explained upon the simple assumption that these animals are endowed with 

 consciousness or a psychical power of some sort. All such reactions, however, 

 may be explained, as in the case of reflex actions from the spinal cord, upon 

 purely mechanical principles, as the necessary response of a definite physical 

 or chemical mechanism to a definite stimulus. To assume that in all cases of 

 this kind conscious processes are involved amounts to making psychical activity 

 one of the universal and primitive properties of protoplasm whether animal 

 or vegetable, and indeed by the same kind of reasoning there would seem to 

 be no logical objection to extending the property to all matter whether living 

 or dead. All such views are of course purely speculative. As a matter of 

 fact we have no means of proving or disproving, in a scientific sense, the exist- 

 ence of consciousness in lower forms of life. To quote an appropriate remark 

 of Huxley's made in discussing this same point with reference to the crayfish, 

 " Nothing short of being a crayfish would give us positive assurance that such 

 an animal possesses consciousness." The study of psychical states in our- 

 selves, for reasons which have been suggested above, does not usually form 

 a part of the science of physiology. The matter has been referred to here 

 simply because consciousness is a fact which our science cannot as yet explain. 

 So far, some of the broad principles of physiology have been considered 

 principles which are applicable with more or less modification to all forms of 

 animal life and which make the basis of what is known as general physiology. 

 It must be borne in mind, however, that each particular organism possesses 

 a special physiology of its own, which consists in part in a study of the 

 properties exhibited by the particular kinds or variations of protoplasm 

 in each individual, and in large part also in a study of the various mechan- 

 isms existing in each animal. In the higher animals, particularly, the com- 

 binations of various tissues and organs into complex mechanisms such as 

 those of respiration, circulation, digestion, or vision, differ more or less in 

 each group and to a minor extent in each individual of any one species. It 



