It may be well just to note the bearing of the principle on the developmentof species. From a dynamic point of view, "natural selection" impliesstructural changes along lines of least resistance. The multiplication ofany kind of plant or animal in localities that are favourable to it, is agrowth where the antagonistic forces are less than elsewhere. And the preservationof varieties which succeed better than their allies in coping with surroundingconditions, is the continuance of vital movements in those directions wherethe obstacles to them are most eluded. §79. Throughout mental phenomena the law enunciated is not readilyestablished. In a large part of them, as those of thought and emotion, thereis no perceptible movement. Even in sensation and action, which show us inone part of the body an effect produced by a force applied to another part,the intermediate movement is inferential only. Some suggestions may be madehowever.
A stimulation implies a force added to, or evolved in, that part of theorganism which is its seat; while a mechanical movement implies an expenditureor loss of force in that part of the organism which is its seat: implyingsome tension of molecular state between the two localities. Hence if, inthe life of a minute animal, there are circumstances involving that a stimulationin one particular place is habitually followed by a contraction in anotherparticular place -- if there is thus a repeated motion through some lineof least resistance between these places; what must be the result as respectsthe line? If this line -- this channel -- is affected by the discharge --if the obstructive action of the tissues traversed, involves any reaction. upon them, deducting from their obstructive power; then a subsequent motionbetween these two points will meet with less resistance along this channelthan the previous motion met with, and will consequently take this channelstill more decidedly. Every repetition will further diminish the resistanceoffered; and thus will gradually be formed a permanent line of communication,differing greatly from the surrounding tissue in respect of the ease withwhich force traverses it. Hence in small creatures may result rudimentarynervous connexions. Only an adumbration of nervous processes thus hintedas conforming to the general law, is here possible. But the effects of associationsbetween impressions and motions as seen in habits, all yield illustrations.
In knitting, in reading aloud, in the performance of the skilled pianistwho talks while he plays, we have examples of the way in which channels ofnervous communication are eventually made so permeable by perpetual dischargesalong them as to bring about a state almost automatic or reflex: illustratingat once the fact that molecular motion follows lines of least resistance,and the fact that motion along such lines, by diminishing the resistance,further facilitates the motion. Though qualifications arising in the samemanner as those indicated in the last chapter complicate these nervo-motorprocesses in ways which cannot here be followed, they do not conflict withthe law set forth. Moreover they are congruous with the principle that inproportion to the frequency with which any external connexion of phenomenais experienced, will be the strength of the answering internal connexionof nervous states. In this way will arise all degrees of cohesion among nervousstates, as there are all degrees of commonness among the surrounding co-existencesand sequences that generate them. Whence must result a general correspondencebetween associated ideas and associated actions in the environment.(*)
The relation between emotions and actions may be similarly construed.
Observe what happens with emotions which are undirected by volitions. Aswas pointed out in the last chapter, there result movements of the involuntaryand voluntary muscles, that are great in proportion as the emotions are strong.
It remains here to add that the order in which these muscles are affectedconforms to the principle. A pleasurable or painful feeling of but slightintensity does little more than increase the action of the heart. Why? Forthe reason that the relation between nervous excitement and cardiac contraction,being common to every species of feeling, is the one of most frequent repetition;that hence the nervous connexion offering the least resistance to a discharge,is the one along which a feeble force produces motion. A stronger sentimentaffects not only the heart but the muscles of the face, and especially thosearound the mouth. Here the like explanation applies; since these muscles,being both comparatively small and, for purposes of speech, perpetually used,offer less resistance than other voluntary muscles to the nervo-motor forces.
By a further increase of emotion the respiratory and vocal muscles becomeperceptibly excited. Finally, under violent passion, the muscles of the trunkand limbs are strongly contracted. The single instance of laughter, whichis an undirected discharge of feeling that affects first the muscles roundthe mouth, then those of the vocal and respiratory apparatus, then thoseof the limbs, and then those of the spine; suffices to show that when nospecial route is opened for it, a force evolved in the nervous centres producesmotion along channels which offer the least resistance, and if is too greatto escape by these, produces motion along channels offering successivelygreater resistance.*
