THE EVOLUTION OF MIND 



ogeneously compounded than any other known 

 systems of molecules ; and the alternate pulling 

 to pieces and putting together of these clusters, 

 which we call nutrition, goes on here with un- 

 paralleled rapidity. Of all known substances, 

 nerve is the most changeable, the most impres- 

 sible, the most readily adaptable to changing 

 combinations of incident forces : in short, the 

 most easily diiferentiable and integrable. Hence 

 we find that those long transit lines, known as 

 afferent and efferent nerves, are not only so 

 constituted that a wave of disturbance set up at 

 one end is immensely increased before it reaches 

 the other end, but are also protected by envel- 

 oping clusters of molecules in such a way that 

 none of the transmitted motion is allowed to es- 

 cape laterally. Ease of transit is here witnessed 

 at its maximum. 



Making use of these theorems of transcen- 

 dental physics, and applying to the problem his 

 vast and accurate knowledge of biological de- 

 tails, Mr. Spencer has propounded a theory of 

 the genesis of nervous systems of all orders of 

 complexity, which, whether entirely or only par- 

 tially true, must be regarded as one of his most 

 brilliant achievements. In the lately published 

 " Physical Synthesis," which concludes the first 

 volume of his " Principles of Psychology," Mr. 

 Spencer shows that the irritability which char- 

 acterizes the entire surface of the lowest ani- 

 213 



