The Kingdom of Man

by Sir E. Ray Lankester

Fig. 56.

Male Trypanosoma Ziemanni, giving rise by nuclear division to eight spermatozoa or microgametes. From the stomach of the gnat (Culex).

Each of these penetrates and fuses with the substance of a female Trypanosome, swallowed at the same time or already taken in by the gnat. The fertilized animalculæ is the vermiform motile stage of Fig. 52, A; and so we return to the starting-point of the cycle (after Schaudinn).

The relations of parasites to the organisms upon or in which they are parasitic, and the relation of man, once entered on the first steps of his career of civilisation, to the world of parasites, form one of the most instructive and fascinating chapters of natural history. It cannot be fully written yet, but already some of the conclusions to which the student is led in examining this subject have far-reaching importance and touch upon great general principles in an unexpected manner.

Before the arrival of man—the would-be controller, the disturber of Nature—the adjustment of living things to their surrounding conditions and to one another has a certain appearance of perfection. Natural selection and the survival of the fittest in the struggle for existence lead to the production of a degree of efficiency and harmonious interaction of the units of the living world, which, being based on the inexorable destruction of what is inadequate and inharmonious as soon as it appears, result in a smooth and orderly working of the great machine, and the continuance by heredity of efficiency and a high degree of individual perfection.

Parasites, whether microscopic or of larger size, are not, in such circumstances, the cause of widespread disease or suffering. The weakly members of a species may be destroyed by parasites, as others are destroyed by beasts of prey; but the general community of the species, thus weeded, is benefited by the operation. In the natural world the inhabitants of areas bounded by sea, mountain, and river become adjusted to one another; and a balance is established. The only disturbing factors are exceptional seasons, unusual cold, wet, or drought. Such recurrent factors may from time to time increase the number of the weakly who are unable to cope with the invasions of minute destructive parasites, and so reduce, even to extermination the kinds of animals or plants especially susceptible to such influences. But anything like the epidemic diseases of parasitic origin with which civilised man is unhappily familiar seems to be due either to his own restless and ignorant activity or, in his absence, to great and probably somewhat sudden geological changes—changes of the connexions, and therefore communications, of great land areas.

It is abundantly evident that animals or plants which have, by long æons of selection and adaptation, become adjusted to the parasites and the climatic conditions and the general company (so to speak) of one continent may be totally unfit to cope with those of another; just as the Martian giants of Mr. H. G. Wells, though marvels of offensive and defensive development, were helpless in the presence of mundane putrefactive bacteria and were rapidly and surely destroyed by them. Accordingly, it is not improbable that such geological changes as the junction of the North and South American continents, of North and South Africa, and of various large islands and neighbouring continents, have, in ages before the advent of man, led to the development of disastrous epidemics. It is not a far-fetched hypothesis that the disappearance of the whole equine race from the American continent just before or coincidently with the advent of man—a region where horses of all kinds had existed in greater variety than in any other part of the world—is due to the sudden introduction, by means of some geological change, of a deadly parasite which spread as an epidemic and extinguished the entire horse population.

Whatever may have happened in past geological epochs, by force of great earth-movements which rapidly brought the adaptations of one continent into contact with the parasites of another, it is quite certain that man, proud man, ever since he has learnt to build a ship, and even before that, when he made up his mind to march aimlessly across continents till he could go no further, has played havoc with himself and all sorts of his fellow-beings by mixing up the products of one area with those of another. Nowhere has man allowed himself—let alone other animals or even plants—to exist in fixed local conditions to which he or they have become adjusted. With ceaseless restlessness he has introduced men and beasts and plants from one land to another. He has constantly migrated with his herds and his horses, from continent to continent. Parasites, in themselves beneficent purifiers of the race, have been thus converted into terrible scourges and the agents of disease. Europeans are decimated by the locally innocuous parasites of Africa; the South Sea islanders are exterminated by the comparatively harmless measles of Europe.

A striking example of the disasters brought about by man’s blind dealings with Nature—disasters which can and will hereafter be avoided by the aid of science—is to be found in the history of the insect phylloxera and the vine. In America the vine had become adjusted to the phylloxera larvae, so that when they nibbled its roots the American vine threw out new root-shoots and was none the worse for the little visitor. Man in his blundering way introduced the American vine, and with it the phylloxera, to Europe; and in three years half the vines in France and Italy were destroyed by the phylloxera, because the European vines had not been bred in association with this little pest, and had not acquired the simple adjusting faculty of throwing out new shoots.