The alternate semi-annual quickening also discloses itself directly in the cartouches; the previous semestral growth from the south polar cap actually showing in them before the impulse from the north began. The slow falling of their curves to the minimum preceding their later rise is nothing less than the dying out of the effect started six months before from the south. The gentler gradient of their fall proclaims a gradual lapse, just as the subsequent sharper rise points to the advent of a fresh impulse. And this deduction seems to be borne out by another circumstance. There is some evidence of decrease in the pre-minimal gradient southward. This is telling testimony to the source whence the impulse came. For if it originated at the south and traveled northward, the southern canals would be the first to be affected and the first to die out, and thus show a longer dead season, exhibited in the cartouches as a more level stretch.
Lastly, the explanation of the canals as threads of vegetation fays in with the one which has been found to meet the requirements of the blue-green areas; while the fact that they prove to develop as they do, reversely to what would take place on earth, is exactly what all we have latterly learnt about the surface conditions of the planet would lead us to expect.
From what has just been said we see that the latest observations at Flagstaff confirm the earlier ones, and, what is especially corroborative, they do so along another line. The former were chiefly static, the latter kinematic. In other words, the behavior of the canals in action bears out the testimony of their appearance at rest.
CHAPTER XXIX
LIFE
Study of the fundamental features of Martian topography has disclosed, as we have seen, the existence of vegetation on the planet as the only rational explanation of the dark markings there, considered not simply on the score of their appearance momentarily, but judged by the changes that appearance undergoes at successive seasons of the Martian year. Thus we are assured that plant life exists on the planet. We are made aware of the fact in more ways than one, but most unanswerably for that trait to which vegetation owes its very name,—its periodic quickening to life. Thus the characteristic which has seemed here most distinctive of this phase of the organic, so that man even christened it in accordance, has proved equally telltale there.
Important as a conclusion this is no less pregnant as a premise. For the assurance that plant life exists on Mars leads to a further step in extramundane acquaintance of far-reaching import. It introduces us at once to the probability of life there of a higher and more immediately appealing kind, not with the vagueness of general analogy, but with the definiteness of specific deduction. For the presence of a flora is itself ground for suspecting a fauna.
Of a bond connecting the two we get our first hint the moment we look inquiringly into the world about us, that of our own earth. Common experience witnesses to a coexistence which grows curious and compelling as we consider it. For it is not confined to life of any special order, but extends through the whole range of organisms of both kinds from the lowest to the highest. Algæ and monera, orchid and mammal, occur side by side and with a certain considerate poverty or richness, as the case may be. Luxuriance in the one is matched by abundance in the other; while a scanty flora means a poor fauna. This of which we have been aware in regions round about us from childhood grows in universality as we explore. Wherever man penetrates out of his proper sphere he finds the same dual possession of the land or the sea, and a similar curtailing or expanding of both tenantries together. No mountain top so cold but that if it grow plants, it supports insects and animals, too, after its kind; no desert so arid but that creeping things find it as possible a habitat as life that does not stir. Even in almost boiling geysers animalcula and confervæ share and share alike. Only where extreme conditions preclude the one do they equally debar the other.
Proceeding now from the fact to its factors we perceive reasons for this tenure in common of the land by the vegetal and animal kingdoms. Examination proves the two great divisions of the organic to be inextricably connected. It strikes our notice first in the relation of plants to animals. It is of everyday notoriety that animals eat plants, though it is less universally understood that in the ultimate they exist on nothing else. Plants furnish the food of animals; not as a matter of partial preference but of fundamental necessity. For the plant is the indispensable intermediary in the process of metabolism. Without plants animals would soon cease to exist, since they are unable to manufacture their own plasm out of the raw material offered by inorganic nature. They must make it out of the already prepared plasm of plants or out of other animals who have made it from plants. So that in the end it all comes back to plant production. The plant is able to build its plasm out of chemical substances; the animal cannot, except in the case of the nitro-bacteria, begin thus at the lowest rung of the alimentary ladder.
But the converse of this dependence is also largely true. Plants are beholden to animals for processes that in return make their own life possible. The latter minister to the former with unconscious service all the time, and with no more arrogant independence than do our domestics generally nowadays. The inconspicuous earthworm is the fieldhand of nature’s crops, who gets his own living by making theirs. Without this day and night laborer the soil for want of stirring had remained less capable of grass. Above ground it is the same story. Deprived of the ministrations of insects many kinds of plants would incontinently perish. By the solicited visits of bees and other hymenoptera—what generically may be classed by the layman as flutter-bys—is the plant’s propagation made possible. Peculiarly well named, indeed, are the hymenoptera, seeing that they are the great matrimonial go-betweens, carrying pollen from one individual to another and thus uniting what otherwise could not meet. Spectacular as this widespread commerce is, it forms but portion of the daily drama in which animals and plants alike take part. From forthright bargainings of honey for help, we pass to less direct but no less effective alliance where plants are beholden to animals for life by the killing of their enemies or the weeding-out of their competitors, and from this to generic furtherance where the interdependence becomes broadcast. In the matter of metabolism the advantage is not all upon one side. In the katabolic process of that which each discards are the two classes of life mutually complimentary,—the waste of the one being the want of the other,—carbonic acid gas being given off by the animal, oxygen by the plant. In biologic economy it is daily more demonstrable that both are necessary constituents to an advancing whole, and that each pays for what it gets by what it gives in return.