TEE PROBLEM OF CONSCIOUSNESS. 297 



another with resemblance. The difference which resembles we term an 

 homology. Our arm, the bird's wing, the lizard's front leg are homolo- 

 gous. The conception of homology both of structure and of function 

 lies at the basis of all biological science, which must be and remain 

 incomprehensible to any mind not thoroughly imbued with this con- 

 ception. Only those who are deficient in this respect can fail to under- 

 stand that the evidence is overwhelming that animals have a conscious- 

 ness homologous with the human consciousness. The proof is conclu- 

 sive. As regards at least mammals — I think we could safely say as 

 regards vertebrates — the proof is the whole sum of our knowledge of 

 the structure, functions and life of these animals. 



As we descend the animal scale to lower forms there is no break 

 and therefore no point in the descent where we can say here animal 

 consciousness ends, and animals below are without it. It seems inevi- 

 table therefore to admit that consciousness extends far down through 

 the animal kingdom, certainly at least as far down as there are animals 

 with sense organs or even the most rudimentary nervous system. It is 

 unsatisfactory to rely chiefly on the anatomical evidence for the answer 

 to our query. We await eagerly results from psychological experiments 

 on the lower invertebrates. A sense organ however implies conscious- 

 ness, and since such organs occur among ccelenterates we are led to 

 assign consciousness to these animals. 



The series of considerations which we have had before us lead 

 directly to the conclusion that the development and improvement of 

 consciousness has been the most important, really the dominant, factor 

 in the evolution of the animal series. The sense organs have been mul- 

 tiplied and perfected in order to supply consciousness with a richer, 

 more varied and more trustworthy store of symbols corresponding to 

 external conditions. The nervous system has grown vastly in com- 

 plexity in order to permit a constantly increasing variety in the time 

 dislocations of sensation. The motor and allied apparatus have been 

 multiplied and perfected in order to supply consciousness with more 

 possibilities of adjustment to external reality which might be advan- 

 tageous. 



If we thus assign to consciousness the leading role in animal evo- 

 lution we must supplement our hypothesis by another, namely, that 

 conscious actions are primary; reflex and instinctive actions secondary, 

 or, in other words, that, for the benefit of the organism, conscious 

 actions have been transformed into reflexes and instincts. Unfortu- 

 nately we must rely chiefly on future physiological and psychological 

 experiments to determine the truth of this hypothesis. Its verification, 

 however, is suggested by certain facts in the comparative physiology 

 of the vertebrate nervous system, which tend to show that in the lower 

 forms (amphibia) a certain degree of consciousness presides over the 



