302 THE RIDDLE OF THE UNIVERSE. 



of the embryonic development of man or any of the 

 higher animals. And from this ontogenetic fact we 

 confidently infer, in virtue of the great biogenetic 

 law, the important phylogenetic proposition, that in 

 the long historical evolution of our ancestors, like- 

 wise, the higher sense-organs with their specific 

 energies were originally derived from the epidermis 

 of lower animals, from a simple layer of cells which 

 had no trace of such differentiated sensilla. 



A particular importance attaches to the circum- 

 stance that different nerves are qualified to perceive 

 different properties of the environment, and these 

 only. The optic nerve accomplishes only the per- 

 ception of light, the auditory nerve the perception of 

 sound, the olfactory nerve the perception of smsll, 

 and so on. No matter what stimuli impinge on and 

 irritate a given sense-organ, its reaction is always of 

 the same character. From this specific energy of the 

 sense-nerves, which was first fully appreciated by 

 Johannes Muller, very erroneous inferences have been 

 drawn, especially in favour of a dualistic and a priori 

 theory of knowledge. It has been affirmed that the 

 brain, or the soul, only perceives a certain condition 

 of the stimulated nerve, and that, consequently, no 

 conclusion can be drawn from the process as to the 

 existence and nature of the stimulating environment. 

 Sceptical philosophy concluded that the very existence 

 of an outer world is doubtful, and extreme idealism 

 went on positively to deny it, contending that things 

 only exist in our impressions of them. 



In opposition to these erroneous views, we must 

 recall the fact that the " specific energy " was not 

 originally an innate, special quality of the various 

 nerves, but it has arisen by adaptation to the 



