244 Heredity. 



arrive at an understanding of thought, we have first to explain 

 how the nervous system is constituted, which is the indispensable 

 condition of all thought. As we are aware, it is only a comple- 

 mentary apparatus : certain infusoria, whose bodies are only an 

 amorphous mass, entirely void of muscles and nerves, have yet 

 a relative life. Relying on the law of evolution, on the passage 

 from the simple to the complex, and on the physiological division 

 of labour, some have endeavoured to explain the genesis of the 

 nervous system. The most curious essays in this direction have 

 been made by one who in other respects rejects the mechanical 

 hypothesis. Mr. Herbert Spencer, in his Biology ( 302), and more 

 particularly in his PsycJiology (Part 5), strives to show how a nerve 

 might be produced in an extremely simple primitive organism 

 by the laws of motion ; and how, from this beginning, more and 

 more complicated nervous systems might be developed. If this 

 bold genesis were beyond question, it would be a great victory for 

 the mechanical theory, but still the necessity would remain of ex- 

 plaining how nerve-vibration becomes a fact of consciousness. 

 We are utterly incapable of understanding how motion becomes 

 thought. The hypothesis is indemonstrable in theory, and incon- 

 ceivable in fact. If it be said that, subjectively, heat and light are 

 as different from motion as the fact of consciousness is different 

 from nerve-vibration, we must observe that the comparison is not 

 exact. For a motion to become light there is need of an optical 

 apparatus and consciousness ; for a motion to become sound there 

 is need of an acoustic apparatus and consciousness. But for a 

 nerve-vibration to become consciousness which as yet has no 

 existence what is needed? How shall we explain this metamor- 

 phosis ? 



Such, briefly, is the mechanical hypothesis, which it would 

 require a volume to set forth in its details. According to it, 

 phenomena differ in nothing from one another save in this, that 

 the higher are produced by a concentration, and the lower by a 

 dispersion of force. A unit of thought would be equivalent to 

 several units of life, and a unit of life to several units of purely 

 mechanical force. At least, such would seem to be the tenour of 

 the observations made by one of its most recent exponents, Dr. 

 Maudsley, in his Physiology of Mind. ' All ascending transform- 



