750 The Relations of Mind and Matter. f August, 
temperature are checked by some sort of protective covering, 
while largely in the Invertebrata, and to some extent in the Ver- 
tebrata, the effects of touch are diminished by a thick armor of 
` scales, horn or shelly matter. In the sessile and inactive forms 
only a small portion of the body is usually exposed to sensory 
influences, the remainder being incased in solid armor through 
which no ordinary impression can penetrate. 
As animals grow superior in organization, however, the need 
of swift and varied motion renders this hard covering a disadvan- 
tage, and it is thrown aside, exposing the whole body to the 
assaults of energy, while the organs of special sense grow con- 
stantly more and more delicate and varied in their susceptibility. 
In these cases, then, the body is invaded by an enormous multi- 
tude of sensory currents, which, could they all reach the muscles, 
would cause a serious loss of physical energy. To check this 
inflow the second method referred to comes into play. The nerve 
currents are forced to traverse ganglia and pass through the fine 
fibrillz of the cells, which, like fine wires in the electric circuit, 
check the current, only permitting a portion of it to pass, while 
the remainder outflows into the ganglion as heat, or is converted 
into some more special mode of energy. 
But this alone would not answer the purpose. In all high 
animal organisms it is necessary that the inflowing energy should 
be under special control. Some power of discrimination must 
exist, or must arise through the action of natural selection, to ` 
decide which currents of sensation shall pass onward to the mus- — 
cles, and which shall be partly or completely checked. Only in 
this way could the nerve currents be prevented from calling up © 
general and indefinite muscular responses, instead of the particu- 
lar and well adapted responses necessary to yield the motions 
demanded by the best good of the economy. This is the third 
element in nerve evolution, and needs now to be considered. 
It is well known that the nerve fiber is a very imperfect con- 
ducting material as compared with the metal of the electric cir- 
_ cuit. How it conducts is an unsettled question. Its current 
resembles that of electricity in being simply a motor influence, 
i perhaps i in both cases set free by chemical change at the starting 
~ point of the circuit. Whether it resembles electricity in being a 
-= Sort of radiant vibration through solid matter cannot be told. All 
* ve know is that its onward movement is excessively slow as 
