62 THE THEORY OF CHANGE OF FUNCTION 



for themselves. An olfactory organ is found in 

 Invertebrates ; some insects almost certainly have 

 them, but these could not have been the ancestors 

 of the vertebrate olfactory organ. Now if the 

 Vertebrates did not inherit them as such they must 

 have got them some other way, either acquired de 

 novo or by change of function. Now in its time of 

 appearance, its mode of development, the histology 

 of its epithelium, there is a very close resem- 

 blance between the olfactory organ and a gill. 

 Hence there are strong reasons for regarding the 

 olfactory organ of Vertebrates as a modified gill. 

 It has been stated that the mode of development of 

 the olfactory nerve as an outgrowth from the 

 cerebral hemisphere constitutes a serious objection to 

 this view as differing from the other cranial nerves. 

 This objection does not now hold good, since it has 

 been shown that the olfactory nerve develops in the 

 same way as other cranial nerves. 



In conclusion, we seem to have in this principle 

 of change of function a real and practicable solution 

 to the chief difficulty in the way of accepting the 

 doctrine of Natural Selection, a solution that has 

 long been overlooked, and whose real importance 

 has, I believe, yet to be appreciated. 



