صور الصفحة
PDF
النشر الإلكتروني

beetles are reared by the ants like their own brood (third step)." In the long run, this leads to the destruction of the ant colony. "Certain native ants even keep the eggs of plant lice with their nests during the winter. The sanguinary robber ant occupies herself almost exclusively with hunting, and leaves the cultivation of plant lice to her slaves." The hunting ants of Africa are accompanied on their forage by "a host of guests, particularly of the family of the short-winged beetles." Other ants make web nests using their own larvae as shuttles. "They conduct the mouth of the larva, from which the spinning substance issues, from one leaf margin to another, and thus weave their nest." 8

A different type of coöperation is seen in the habits of such birds as cowbirds in America or the herons which accompany the herds of buffalo in Africa. Here these companions either feed on the parasites of their fourfooted friends or take advantage of the insects attracted by their presence or disturbed by their movements through the grass.

Curious indeed are many of these relationships. Certain of the minute mussels attach themselves temporarily to some fresh water fish like the minnow, later dropping off far from the starting point; while perhaps, to reverse the relationship, the young-of one fish at least, the bitterling, are for a time parasites in the gills of a mussel." A more direct dependence exists in the case of the parasites which infest the fur or feathers of large animals. All birds and mammals continually carry with them such insects as lice, fleas and ticks, which draw their nourishment from their unwilling hosts. Though this process

8 Annual Report of Smithsonian Institute, 1912, p. 455. 9 THOMPSON, J. A. o. c., p. 55.

is often irritating the burden is usually small, save when the parasites are the carriers of disease. Even man himself is rarely free from such companions until he reaches a relatively high standard of personal cleanliness while the lower ranks of society are continually infested. To this parasitic class belong a few forms of plant life such as the fungus producing the disease known as thrush.

Much more serious on the whole from the standpoint of the welfare of the invaded organism are the internal parasites, whether plant or animal. These are now recog

nized as the causes of some of the chief diseases to which flesh is heir. The story of man's attempt to conquer these invaders will be told in the next chapter. Here the fact must be noted and its significance indicated. These parasites vary in size from the minute one-celled formssome of which are probably too small to be seen with the strongest microscopes; some that we have not yet isolated, although the results of their presence are only too well known to the great tapeworm which occasionally dwells in our intestines. We know three forms of the bacteria, of which the smallest are the cocci, some 1/150,000 of an inch in diameter. The rodlike forms (bacilli) are from 1/25,000 to 1/4,000 of an inch in length by 1/125,000 to 1/6,000 of an inch in diameter; while the largest spirilli are about 1/600 of an inch in length. One drop of sour milk may contain 40,000,000 bacteria. Some of these are never found in man but are common among animals as, for instance, the cholera of chickens; others, like measles, chicken-pox and typhus fever so far as we know are peculiar to man; while many others, diphtheria, tuberculosis and anthrax are common to man and animals. We now know that some organisms like those causing malaria and yellow fever pass one part of their life cycle

[ocr errors]

in the bodies of the mosquito or other insects and by them are transferred to man. In all such cases, the lower forms of life live at the expense of the higher forms and not infrequently destroy them. Plants parasitic upon others as the mistletoe on the oak are also common. The illustrations already given make it clear that this series of relationships has many sides and may be considered from several viewpoints. The relations of the bee and the clover are mutually advantageous. The dog carrying the cocklebur performs an unrequited service. From the standpoint of the mouse his relations to the cat are decidedly one-sided, and man is far from pleased at being the host of disease germs. Now these two aspects of the "web of life" may be called "mutual aid,” or coöperation, and "the struggle for existence," or competition; both of which are of such tremendous significance to the student of life, though in the last century the emphasis on the latter phase has largely obscured the existence of the former. We thus have a paradox. Life depends upon life and this involves the destruction of life.

Communities rather than isolated individuals are characteristic of animals. Plant associations are accidental rather than voluntary, but animals seem to prefer the presence of their fellows. "Whether the feeling be fear, experienced at the appearance of a bird of prey, or a 'fit of gladness,' which bursts out when the animals are in good health and especially when young, or merely the desire of giving play to an excess of impressions and vital power the necessity of communicating impressions, of playing, of chattering, or of simply feeling the proximity of other kindred living beings pervades Nature, and is as much as any other physiological function, a distinctive

[ocr errors]

feature of life and impression ability. This need takes a higher development and attains a more beautiful expression in mammals, especially amidst their young, and still more among the birds; but it pervades all Nature.” 10

Whenever man has entered a relatively unpopulated continent where conditions were favorable he has been amazed at the abundance of wild life. "I found the Cossacks in the villages of that gorge in the greatest excitement because thousands and thousands of fallow deer were crossing the Amur where it is narrowest in order to make the lowlands." 11 "For several hundred yards from the shore the air is filled with gulls and terns, as with snowflakes on a winter day. Thousands of plover and sand coursers run over the beach, searching for their food, whistling and simply enjoying life. Further on, on almost each wave, a duck is rocking, while higher up you notice the flocks of the Casarki ducks. Exuberant life swarms everywhere." 12

Men now living can recall the enormous herds of bison that roamed the Western prairies, the passenger pigeons whose vast flocks almost darkened the sun and broke the branches of the trees on which they settled or the armies of squirrels. In the far north the caribou still migrate in companies of thousands. Though of a younger generation, the writer has been privileged to see some of the rocky islands so densely covered with birds that one thought the surface moved when they took wing. He has seen prairie chickens leaving the wheat stubble of Dakota by the hundreds and has watched in early spring great companies engaged in lovemaking. Birds ordinarily considered as rather solitary often combine in large flocks. 10 KROPOTKIN, P. Mutual Aid, p. 55.

11 Ibid., p. 48. 12 Ibid., p. 33.

The great winter flocks of crows going to and from their roosting places to the feeding grounds are common sights. I have witnessed some remarkable flights of hawks when for several consecutive days individuals of many species drifted lazily by, scores being constantly in sight. I have known the short-eared owls to gather in large companies. Blind indeed is he who has not viewed with wonder the migrations of the birds in spring and fall. Who has not awakened some spring morning to find the fields and woods full of a variety of birds of which a day before only a few could be found? Who has not been thrilled by the melody of the bobolink or admired the formal columns of the geese? These migrating groups are often of very distinct species. Eight kites, one crane and one peregrine falcon are reported as forming one motley group.

It must not be thought that these are always chance associations. There are many illustrations of a definite purpose. Pelicans "always go fishing in numerous bands and after having chosen an appropriate bay, they form a wide half-circle in face of the shore, and narrow it by paddling towards the shore, catching all the fish that happen to be inclosed in the circle. On narrow rivers and canals they even divide into two parties, each of which draws up on a half-circle, and both paddle to meet each other, just as if two parties of men dragging two long nets should advance to capture all fish taken between the nets when both parties come to meet." 13 Hunting parties of animals are well known. Monkeys combine to get food. Brazilian kites are said to summon assistance if the prey is too large. Kingbirds frequently combine to chase a crow or hawk. If a burying beetle discovers a dead mouse it summons from four to ten others to help. Crabs have 13 KROPOTKIN, P. o. c., p. 23.

« السابقةمتابعة »