ORGANS OF SENSE. 235 



evolution proves most clearly that this received conception 

 is radically false. The history of evolution convinces us that 

 the highly purposive and admirably constituted sense organs, 

 like all other organs, have developed without premeditated 

 aim ; that they originated by the same mechanical process 

 of Natural Selection, by the same constant interaction 

 of Adaptation and Heredity, by which all the other pur- 

 posive contrivances of the animal organization have been 

 slowly and gradually evolved during the " Struggle for 

 Existence." 



Like most other Vertebrates, Man possesses six distinct 

 organs of sense, which accomplish seven distinct sensations. 

 The external skin-covering accomplishes the sensation of 

 pressure (resistance) and of temperature (warmth and cold). 

 This is the earliest, the lowest, and the least differentiated 

 organ of sense ; it is distributed over the entire surface of 

 the body. The other sensorial activities are localized. The 

 sexual sense is limited to the skin-covering of the external 

 sexual organs, just as the sense of taste is limited to the 

 mucous membrane of the mouth-cavity (tongue and palate), 

 and the sense of smell to the mucous membrane of the 

 nose-cavity. Special mechanical contrivances of great com- 

 plexity exist for the two highest and most differentiated 

 organs of sense, the eye for the sense of sight, and the ear 

 for that of hearing. 



Comparative Anatomy and Physiology show that in the 

 low animals specialized sense-organs are entirely wanting, and 

 that all sensations are transmitted through the outer surface 

 of the skin-covering. The undifferentiated skin-layer, or exo- 

 derm, of the Gastrsea is the simple cell-layer from which the 



