Again, accompanying the structural differentiations and integrations ofthe alimentary canal, there arise differentiations and integrations bothof its mechanical movements and its actions of a non-mechanical kind. Alongan alimentary canal of a primitive type there pass, almost uniformly fromend to end, waves of constriction. But in a well-organized alimentary canal,the waves of constriction are widely unlike at different parts, in theirkinds, strengths, and rapidities. In the oesophagus they are propulsive intheir office, and travelling with considerable speed, take place at intervalsduring eating, and then do not take place till the next meal. In the stomachanother modification of this originally uniform action occurs: the muscularconstrictions are powerful, and continue during the long periods that thestomach contains food. Throughout the upper intestines, again, a furtherdifference shows itself -- the waves travel along without cessation but arerelatively moderate. Finally, in the rectum this rhythm departs in anotherway from the common type: quiescence, lasting for many hours, is followedby a series of strong contractions. Meanwhile, the essential actions whichthese movements aid, have been growing more definitely heterogeneous. Secretionand absorption are no longer carried on in much the same way from end toend of the tube; but the general function divides into various subordinatefunctions. The solvents and ferments furnished by the coats of the canaland the appended glands, become widely unlike at upper, middle, and lowerparts of the canal; implying different kinds of molecular changes. Here theprocess is mainly secretory there it is mainly absorbent, and in other places,as in the oesophagus, neither secretion nor absorption takes place to anyappreciable extent. While these and other internal motions, sensible andinsensible, are being rendered more various, and severally more integratedand more distinct, there is advancing the integration by which they are unitedinto local groups of motions and a combined system of motions. While thefunction of alimentation subdivides, its subdivisions become co-ordinated,so that muscular and secretory actions go on in concert, and so that excitementof one part of the canal sets up excitement of the rest. Moreover, the wholealimentary function, while it supplies matter for the circulatory and respiratoryfunctions, becomes so integrated with them that it cannot for a moment goon without them. And, as evolution advances, all three of these fundamentalfunctions fall into greater subordination to the nervous functions -- dependmore and more on the due mount of nervous discharge; while at the same timetheir motions become co-ordinated, or in a sense integrated, with those ofthe nervo-muscular system, on which they depend for the supply of materials.
When we trace up the functions of motor organs the same truth disclosesitself. Microscopic creatures are moved through the water by the oscillationsof cilia, here large and single or double, and here smaller and numerous;and various larger forms, as the Turbellaria, progress by ciliary actionover solid surfaces. These motions of cilia are, in the first place, severallyvery minute; in the second place they are homogeneous; and in the third placethere is but little definiteness in them individually, or in their jointproduct, which is mostly a random change of position not directed to anyselected point. Contrasting this ciliary action with the action of developedlocomotive organs, we see that instead of many small or unintegrated movementsthere are a few comparatively large or integrated movements; that actionsall alike are replaced by actions partially or wholly unlike; and that insteadof being very feebly or almost accidentally co-ordinated, their definiteco-ordination renders the motions of the body as a whole, precise. A parallelcontrast, less extreme but sufficiently decided, is seen when we pass fromthe lower types of creatures with limbs to the higher types of creatureswith limbs. The legs of a Centipede have motions that are numerous, small,and homogeneous; and are so little integrated that when the creature is dividedand subdivided, the legs belonging to each part propel that part independently.
But in one of the higher Arthropoda: as a Crab, the relatively few limbshave motions which are comparatively large in their amounts, which are considerablyunlike one another, and which are integrated into total bodily movementsof much definiteness. §143. The last illustrations introduce us to illustrations of thekind classed as mental. They are the physiological aspects of the simpleramong those functions which, under a more special and complex aspect, wedistinguish as psychological. The phenomena subjectively known as changesin consciousness, are objectively known as nervous excitations and discharges,which science now interprets into modes of motion. Hence, in following uporganic evolution, advance of the retained motion alike in integration, inheterogeneity, and in definiteness, may be expected to show itself both inthe visible nervo-muscular actions and in the correlative mental changes.
We may conveniently look at the facts as exhibited during individual evolution,before looking at them as exhibited in general evolution.
The progress of a child in speech very clearly displays the transformation.
