22 THE EVOLUTION OF MAN. 



This important relation is most clearly seen in th<s 

 history of the evolution of the nervous system. In the 

 economy of the human body, this system performs the func- 

 tions of sensation, of voluntary movement, volition, and 

 finally the highest psychical functions, namely, those of 

 thought ; in a word, every one of the various activities which 

 constitute the special subject of Psychology, or the science 

 of the mind. Modern Anatomy and Physiology have demon- 

 strated that these functions of the mind, or psychic activities, 

 are immediately dependent upon the more delicate structure 

 of the central nervous system, upon the internal conditions 

 of the form of the brain and the spinal marrow. Here 

 are placed the extremely complex mechanism of cells, whose 

 physiological function constitutes the mind-life of Man. 

 It is so complex that to most people its function appears 

 to be something supernatural, and incapable of mechanical 

 explanation. But the history of the evolution of the in- 

 dividual furnishes us with the most surprising and signi- 

 ficant information as to the gradual origin and progressive 

 formation of this most important system of organs. For the 

 first rudiment of the central nervous system in the human 

 embryo makes its appearance in the same most simple form 

 in which Ascidians and other inferior Worms retain it 

 throughout life. A perfectly simple spinal marrow, without 

 brain, such as throughout its existence represents the organ 

 of the mind of the Amphioxus, the lowest of Vertebrates, 

 first develops from this rudiment. It is only at a later 

 period that a brain develops from the anterior extremity 

 of this spinal cord, and this brain is of the simplest form, 

 •similar to the permanent form of this organ in the lower 

 Fishes. Step by step this simple brain develops still 

 further, passing through forms corresponding to those of 



