standing and interpretation of many of the cases which will be found in nature is extremely hard and moral maxims based on natural phenomena are of questionable worth. Nature cares little for the hard and fast boundary lines which man is always trying to establish.

Symbiosis

In general, symbiosis implies a permanent association between two forms of life with "contrasting methods of feeding," as Lull says.

A form of Amoeba and many radiolarians among the Protozoa or unicellular animals, together with the fresh-water polyp, have numerous unicellular green plants, which live in the protoplasm of the animal. From the animal the plants receive protection and certain wastes, which they utilize as food. In return for this the animal receives oxygen, given off by the plants during the formation of starches.

But a still more remarkable instance lies in the lichens, which are compound plants formed by the intimate union of a fungus, which contributes to the partnership protection, attachment, and raw food materials, air, water, and nitrogenous salts, and a green alga, which elaborates the food. They can live alone, but the combination is much better fitted to withstand the vicissitudes of life than either would be singly.1

Commensalism

Commensalism is common.

Fishes are found in association with sea anemones, being in some way immune either to the sting or to the digestive juices of the creature within whose internal cavity they dwell. One fish dwells in amicable association with a sea cucumber, lying in the rectum of the latter, whence it darts out to secure its prey. A curious adaptation of the fish lies in the shifting forward of its own vent to a position beneath the throat, so that when it lies, as it often does, with its anterior portion protruding from its host the latter is not defiled by the voided waste of the fish. It is difficult to see what, if any, advantage is gained by the sea cucumber in this strange partnership, but it is not yet parasitism,

because, so far as we know, no harm is done to the host. Another similar association is that of the remora, or sucking fish, and the shark. The former has an amazing modification of the dorsal fin into a broad, cellular, sucking disk on the top of the head, by means of which it adheres to the larger fish. A quick detachment is made when prey is sighted, and as quick a return when its hunger is satisfied. Here again the benefit to the shark or other towing fish is doubtful; as a matter of fact its speed must be markedly retarded. Whether it is a welcome guest and thus fills the need of companionship to the shark, or whether, like some well-meaning friends, it is merely "put up with," is by no means clear."

Parasitism

Parasitism is so common and well known that illustrations are scarcely needed. One has but to recall the vast hosts of fleas, ticks, and mosquitoes, the invading armies of disease germs, the liver worms and tapeworms, to realize that all the higher forms of life are perennial hosts of parasites, many of which cause some harm and often kill the host. The "web of life" which nature has spun is so intricate that no one man may hope to understand it all.

REPOPULATION OF DEVASTATED AREAS

Once in a while we have a chance to see how nature starts anew to spin its web. The volcanic eruption at Kratakoa in 1883 covered two islands with hot ashes and stones, destroying all life. Three years later they were visited by the director of the botanic gardens of Java, M. Traub. “He made the remarkable discovery that the bare surface of the eruptive matter belched forth by the volcano was carpeted by a thin, slimy layer; it proved to consist of blue algae, which are coated with a viscous layer that easily retains water, and so already contributes to the disintegration, the breakingup and weathering of the volcanic matter. At the same time the moist coating favored the development of fern-spores which were not slow to avail themselves of the chance. Both

the blue algae and the fern-plants were blown across the sea as spores by the wind." In time the islands will support much vegetation and the animals will find their way there again.

HOW ANIMALS SERVE PLANTS

In partial return to the vegetable world for the food it supplies to animals, the latter make contributions of no small value to plants. Probably the chief of these services is in the distribution of the pollen not only from one plant to another but also for many flowers where direct fertilization is impossible without the assistance of insects.

More than two thousand years ago Herodotus observed a curious custom in Egypt. At a certain season of the year, the Egyptians went into the desert, cut off branches from the wild palms, and bringing them back to their gardens, waved them over the flowers of the date palm. Why they performed this ceremony they did not know; but they knew that if they neglected it, the date crop would be poor or wholly lost. Herodotus offers the quaint explanation that along with these branches there came from the desert certain flies possessed of a "vivific virtue" which somehow lent an exuberant fertility to the dates. But the true rationale of the incantation is now explained. Palm trees, like human beings, are male and female. The garden plants, the date bearers, were females; the desert plants were males; and the waving of the branches over the females meant the transference of the fertilizing pollen dust from the one to the other.'

In nature either wind or insects must carry the fertilizing pollen.

In the case of the red clover, bumblebees are the agents. Thomson in happy fashion enlarges one of Darwin's illustrations.

"Plants and animals remote in the scale of nature are bound together by a web of complex relations. . . . I have also found that the visits of bees are necessary for the fertilization of some kinds of clover, thus 100 heads of red clover produced 27,000

seeds, but the same number of protected heads produced not a single seed. Humble bees alone visit red clover, as other bees cannot reach the nectar. . . . We know that the red clover, importd to New Zealand did not bear fertile seeds till humble bees were also imported. The number of humble bees in any district depends in great measure on the number of field mice which destroy their combs and nests; and Colonel Newman, who has long attended to the habits of humble bees, believes that more than two-thirds of them are thus destroyed all over England." Now the number of mice is largely dependent, as every one knows, on the number of cats; and Colonel Newman says: "Near villages and small towns I have found the nests of humble bees more numerous than elsewhere and this I attribute to the number of cats that destroy the mice." Thus we may say with Darwin that next year's crop of purple clover is influenced by the number of humble bees in the district, which varies with the number of the field mice, that is to say with the number of cats.3

Commenting on Darwin's original observation Huxley humorously said that every one knew that the number of cats varied with the number of old maids. The amount of labor performed by the bees is enormous. A red clover blossom contains less than one-eighth grain of sugar. There are 7,000 grains in a pound. The bees must visit some 56,000 clover heads, each having some 60 florets, thus repeating the operation 3,360,000 times, to get a pound of honey.

Most of the pollenization is due to the chance contact of the insect. In some instances the insect deliberately moves the pollen for its own purposes and thus unwittingly brings it in contact with the pistils of the flower. This is the case with the yucca of our Southwest which depends solely upon the little yucca moth. The fig industry of California was made possible by the importation in 1900 of an insect which fertilizes the blossoms of the Smyrna figs.

Many animals besides man assist in the distribution of plants. Seeds are dropped by the birds along the fence rows. Squirrels bury many nuts which they fail to recover later and which may thus get chance to grow. Burrs catch in

the fur of passing animals and are transported over considerable distances. A similar service is also performed by the larger animals for the smaller. Minute mollusks attach themselves to the feet of birds and drop off in some new location which may or may not be favorable. If it is, a new colony is formed. Cattle transported from the Southwest carried the ticks which caused the dreaded Texas fever in the northern herds. The story of man's activity will be told in the next chapter.

RATE OF REPRODUCTION

In as much as the life of any given organism is limited, although the limits may vary from an hour for some bacteria to a couple of thousand years for some trees, nature has to provide some plan of reproduction if any species is to survive. As a whole the lowest forms of life multiply most rapidly. A cholera bacillus may divide every twenty minutes and might thus in one day become 5,000,000,000,000,000,000,000 with an estimated weight of 7,366 tons. An annual plant with only two seeds would be represented by 1,048,576 in twenty-one years. The female of the house fly lays 120 eggs and has twelve generations a season at Washington, D. C. It breeds chiefly in horse manure where it has been found at the rate of 1,200 to the pound. The descendants of a single pair between April and November might amount to 214,577,844,320,000,000,000,000, while the offspring of a pair of mosquitoes in 180 days would be represented by the figure 2 at the left of one of these lines followed by a full row of ciphers. A herring has been known to have 47,000 eggs, a 75-pound cod has 9,100,000 eggs and a giant ocean sunfish (Mola) is said to have contained 300,000,000 eggs. A pair of robins having four young each season, assuming that the descendants lived and reproduced at the same rate, would have 20,913,948,846 descendants at the end of twenty years. The entire present human race