Animals assist greatly in the spread of plants and trees. Along the fence rows appear cherries, sumac, dogwood and junipers whose seeds have been deposited by birds. Squirrels often carry walnuts, chestnuts and acorns considerable distances. Many seeds like burdock or cockleburs catch in the hair of passing animals and are thus transported to new locations.

Every observer has in spring time watched the ants crawling over the buds of flowers and at first doubtless feared lest they do them damage. Closer study shows that the ants are seeking the aphids or plant lice from whose bodies they get a drop of liquid of which they appear very fond, sometimes stroking and caressing the aphid to stimulate the production of the fluid. Hence aphids have been called the slaves or the "cows," as Linnæus names them, of the ants. In any case they directly furnish food or some acceptable stimulant.

The complex social life of bees and ants is so often described that any extended account is unnecessary. It is worth while to indicate some of their methods, however, which vary greatly according to the species. The Amazon ant "exists only by the capture of slaves and in that connection develops the most brilliant warrior talent that we know in the entire animal kingdom. Its mandibles are modified to be solely weapons for killing and are unsuited for domestic occupations; furthermore it has even lost the instinct of feeding by itself and must be fed out of the mouths of its slaves." Wasmann estimates that some 2,000 species of animals are found in association with ants, some as guests, some as slaves, others as parasites. Some beetles "are not only licked by their hosts (first step), but are also fed regularly from their mouths (second step), and finally also the larvæ of these

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.9 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 recognized 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 forms— some 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

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

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.