CH. XVI.] THE EVOLUTION OF MIND. 145 



line of traction. A good illustration is afforded by the 

 f^radual evolution of the circulatory system as we ascend in 

 the animal scale. In the lowest animals which possess any 

 nutritive fluid perceptibly distinct from the protoplasmic 

 jelly of which their bodies are composed, this fluid percolates 

 here and there at seemiufr random, its course being determined 

 by local pressures, just as in the case of rain-water trickling 

 through the ground. Now as we ascend to higher animals, 

 we find that the nutritive fluid has wrought for itself certain 

 channels, to which it confines itself, and which gradually 

 become more and more definite in direction, and more and 

 more clearly demarcated from the adjacent portions of tissue. 

 Until, when we reach animals of a high type of structure, we 

 find the blood coursing through permanent channels, the 

 walls of which contract and expand in such a way as to 

 assist the blood in its progress. A similar explanation is 

 to be given of the genesis of the contractile fibres of muscle, 

 as due to the continuance of molecular undulations along 

 certain lines. 



When we come to the nervous system, we find most com- 

 pletely realized all the conditions requisite for the rapid 

 establishment of permanent transit-lines. The clusters of 

 molecules of which nerve-tissue is composed, are more 

 heterogeneously compounded than any other known systems 

 of molecules ; and the alternate pulling to pieces and put- 

 ting together of these clusters, which we call nutrition, goes 

 on here with unparalleled rapidity. Of all known sub- 

 stances, nerve is the most changeable, the most impressible, 

 the most readily adaptable to changing combinations of 

 .ncident forces, — in short, the most easily differentiable and 

 integrable. Hence we find that those long transit-lines, 

 known as afferent and efferent nerves, are not only so con- 

 stituted that a wave of disturbance set up at one end is 

 immensely increased before it reaches the other end, but are 

 ftlso protected by enveloping clusters of molecules in such a 



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