450 THE EVOLUTION OF MAN. 



Turning now from observing the comparative mental 

 capacity of the various animals to the question as to the 

 organs of these functions, we receive the answer, that in all 

 higher animals they are invariably connected with certain 

 groups of cells, those cells which compose the central 

 nervous system. All naturalists, without exception, agree 

 that the central nervous system is the organ of the mental 

 life of animals, and this assertion is at any time capable 

 of experimental proof. If the central nervous system is 

 wholly or partially destroyed, the " mind," or the psychical 

 activity of the animal, is wholly or partially annihilated at 

 the same time. We must, therefore, next inquire what is 

 the character of the mental organ in man. The undeniable 

 answer to this question has already been given. Man's 

 mental organ is, in its whole structure and origin, the same 

 as that of all other Vertebrates. It originates as a simple 

 medullary tube from the outer skin of the embryo, from 

 the skin-sensory layer, or the first of the secondary germ- 

 layers. In the course of its gradual development it passes 

 through the same stages of progression in the human 

 embryo as in that of all other Vertebrates, and as these 

 latter have undoubtedly a common origin, so must also the 

 brain and spinal cord be of the same origin in all. 



Physiological observation and experiment teaches, more- 

 over, that the relation of the " mind " to its organ, the brain 

 and spinal marrow, is exactly the same in Man as in all 

 other Mammals. The former can in no case act without 

 the latter; the one is connected with the other, as is 

 muscular movement with muscle. Therefore, the mind can 

 develop only in connection with its organ. Adherents of 

 the Theory of Descent, who concede the causal connection 



