230 INITIATIVE IN EVOLUTION 



As to the scratch reflex in the dog it was only when it was 

 easily elicitable that it could be evoked by electrical stimulation as 

 well as mechanical, and when it was not easily elicitable electrical 

 stimuli failed altogether while mechanical stimuli still evoked it. 



He describes the receptor as a mechanism " attuned to respond 

 specially to a certain one or ones of the agencies that act as 

 stimuli to the body," and points to the fact that electrical stimuli 

 are not of common occurrence in nature and no chance for adaptation 

 to evolve in the organism receptors appropriate for such stimuli 

 has been afforded. Such negative facts are at the least suggestive 

 in considering the question of the mode of origin of receptors and 

 end-organs, electrical stimuli being rare in nature. 



The subject of the innervation of the skin and its receptors 

 has been treated here in a great measure by the aid of imagination, 

 with some evidence, and a good deal of reconstruction has been 

 attempted, but perhaps this will be pardoned by those who are 

 prepared to carry out a corresponding process with such as Pithe- 

 canthropus, Eoanthropus and Saurian monsters from somewhat 

 scanty osseous remains. Any biological theory of the origin of 

 these receptors than the one here put forward is faced with some 

 formidable difficulties, which are probably insurmountable. 



