2 DIFFICULTY OF THE SUBJECT. 



but have selected tliose cases which seemed to me the 

 most suggestive, interesting, and instructive. 



No one can doubt that the sensations of other animals 

 differ in many ways from ours. Their organs are some- 

 times constructed on different principles, and situated 

 in very unexpected places. There are animals which 

 have eyes on their backs, ears in their legs, and sing 

 through their sides. Nevertheless, in considering the 

 different senses, it will probably be most convenient to 

 begin by a short summary of our own organs, as afford- 

 ing the best clue to the purposes and functions of cor- 

 responding structures among the lower animals. The 

 subject is one of very great difficulty. Even as regards 

 our own senses, we are still in extreme ignorance. The 

 clue afforded by anatomy is very imperfect, and some- 

 times almost misleading. No one can read the literature 

 relating to the organs of sense without feeling how 

 very little we really know on the subject. Even when, 

 as especially in the cases of the organs of hearing and 

 sight, we have careful and elaborate descriptions and 

 figures of very complex structures, these relate rather 

 to the separation and arrangement of the waves of sound 

 or light, than to the actual manner in which they affect 

 the nervous system itself ; while as to the manner in 

 which our perceptions are in turn created, we are almost 

 absolutely ignorant. In the senses of taste and smell 

 this becomes, perhaps, even more clearly evident. 



Every cell, indeed, in the animal body is a standing 

 miracle. Consider what it has to do. It must grow ; it 

 must assimilate nourishment ; it must secrete ; it must 

 produce other cells like itself; and this often in addition 

 to its own proper and distinctive function. The lowest 

 animals consist but of a single cell. Yet they feed and 



