There is some evidence that modifications in the Earth's crust due toigneous action have an indefinite periodicity. Volcanic eruptions are notcontinuous but intermittent, and as far as the data enable us to judge, havesomething like an average rate of recurrence, as witness the case of Kilauea;which rate is complicated by rising into epochs of greater activity and fallinginto epochs of comparative quiescence. So too, according to Mallet, is itwith earthquakes and the elevations or depressions caused by them. Sedimentaryformations yield indirect evidence. At the mouth of the Mississippi the alternationof strata gives decisive proof of successive sinkings of the surface, thathave taken place at tolerably equal intervals. Everywhere in the extensivegroups of conformable strata that imply small subsidences recurring witha certain average frequency, we see a rhythm in the action and reaction betweenthe Earth's crust and its contents -- a rhythm compounded with those slowerones shown in the termination of groups of strata, and the commencement ofother groups not conformable to them. §85. Perhaps nowhere are illustrations of rhythm so numerous andso manifest as among the phenomena of life. Plants do not, indeed, usuallyshow us any decided periodicities, save those determined by day and nightand by the seasons. But in animals we have a great variety of movements inwhich the alternation of opposite extremes goes on with all degrees of rapidity.
The swallowing of food is effected by a wave of constriction passing alongthe oesophagus; its digestion is largely aided by a muscular action of thestomach that is also undulatory; and the peristaltic motion of the intestinesis of like nature. The blood obtained from this food is propelled in pulses,and is aerated by lungs that alternately contract and expand. All locomotionresults from oscillating movements. Even where it is apparently continuous,as in many minute forms, the microscope proves the vibration of cilia tobe the agency by which the creature is moved smoothly forwards.
Primary rhythms of the organic actions are compounded with secondary onesof longer duration. We see this in the periodic need for food, and in theperiodic need for repose. Each meal induces a more rapid rhythmic actionof the digestive organs; the pulsation of the heart is accelerated; the inspirationsbecome more frequent. During sleep, on the contrary, these several movementsslacken. So that in the course of the twenty-four hours, those small undulationsof which the different kinds of organic action are constituted, undergo onelong wave of increase and decrease, complicated with several minor waves.
Experiments have shown that there are still slower rises and falls of functionalactivity. Waste and assimilation are not balanced by every meal, but oneor other maintains for some time a slight excess; so that a person in ordinaryhealth undergoes an increase and decrease of weight during recurring intervalsof tolerable equality. There are oscillations of vigour too. Even men intraining cannot be kept stationary at their highest power, but when theyhave reached it begin to retrograde. Further evidence of rhythm in the vitalmovements is furnished by invalids. Sundry disorders are named from the intermittentcharacter of their symptoms. Even where the periodicity is not very marked,it is mostly traceable. Patients rarely if ever become uniformly worse; andconvalescents have usually their days of partial relapse or of less decidedadvance.
Aggregates of living creatures illustrate the general truth in other ways.
If each species of organism be regarded as a whole, it displays two kindsof rhythm. Life as it exists in every member of such species, is an extremelycomplex kind of movement, more or less distinct from the kinds of movementwhich constitute life in other species. This extremely complex kind of movementbegins, rises to its climax, declines, and ceases in death. And every individualin each generation thus exhibits a wave of that peculiar activity characterizingthe species as a whole. The other form of rhythm is seen in that variationof number which each tribe of animals and plants undergoes. Throughout theunceasing conflict between the tendency of a species to increase and theantagonistic tendencies, there is never an equilibrium: one always predominates.
In the case even of a cultivated plant or domesticated animal, where artificialmeans are used to maintain the supply at a uniform level, oscillations ofabundance and scarcity cannot be avoided. And among creatures uncared forby man, such oscillations are usually more marked. After a race of organismshas been greatly thinned by enemies or innutrition, its surviving membersbecome more favourably circumstanced than usual. During the decline in theirnumbers their food has grown relatively abundant, while their enemies havesomewhat diminished from want of prey. The conditions thus remain for sometime favourable to their increase, and they multiply rapidly. By-and-by theirfood is rendered relatively scarce, at the same time that their enemies havebecome more numerous; and the destroying influences being thus in excess,their number begins to diminish again. Yet one more rhythm, extremely slow,may be traced in the phenomena of Life under their most general aspect. Theresearches of palaeontologists show that there have been going on, duringthe vast period of which our sedimentary rocks bear record, successive changesof organic forms. Species have appeared, become abundant, and then disappeared.