64 PRINCIPLES OF PHYSIOLOGY 



retinal elements of the eye of a codfish are 

 very different from those in the human retina. 

 Little attention has yet been paid to the 

 evolution of tissues, and the question of 

 whether it has been brought about by the 

 agency of external circumstances on a particu- 

 lar tissue, or by the influence of one tissue on 

 another, cannot yet be answered. It is strik- 

 ing, however, to observe that evolution, which 

 depends on physiological processes, affects not 

 merely the outward form but even the tissues 

 that build up the body of an animal. 



