might have descended from a single pair starting at the year one of our own era.

Perhaps one of the most remarkable experiments that have thus far been carried out with the lower forms of life is that of Professor Woodruff of Yale, with the slipper animalcule, the protozoan Paramecium. Woodruff began his pedigreed culture from a wild individual on May 1, 1907. They reproduce by simple division about three times in forty-eight hours, so that by May, 1912, they had reached 2,945 generations, with a potential bulk of protoplasm ten thousand times that of the earth. And the individual is on the verge of invisibility! It has been further calculated that at the nine-thousandth generation—and they have passed that to-day, for they average some six hundred plus a year-the resultant protoplasmic mass would exceed the confines of our universe and would be extending into space with the rapidity of light.9

Now, in the actual world no such increase as indicated by any of the illustrations takes place. Yet every one knows that there is a most rapid increase at times. There is a vast difference in the number of flies in early spring and in the autumn. Eight male and seven female large-mouthed black bass were put by the Fish Commission in a pond at Washington, D. C., in May, 1892. At the end of November, 37,000 young measuring from 3 to 4 inches, and 500 weighing about a half pound each were removed. 10 It is evident that there is a rapid replacement of adults by the young and a seasonal, if not permanent, increase in the total number of any species. The total aggregate of life can neither be measured nor comprehended. A drop of sour milk may contain 40,000,000 bacteria. About all that we can do is to say that there appears to be as much life in every part of earth as existing conditions make possible, granting that conditions may change, and then try to discover the conditions.

THE BALANCE OF LIFE

The clew to the riddle lies in the fact that every organism requires food and, as already stated, in most instances this

involves the destruction of other organisms. No organic product is known which is not eaten by some insect.

If all the feeders on vegetable life were allowed to develop absolutely without check during two successive years, the first of them would see every green thing swept from the face of the earth, and the second would destroy all possibility of the recurrence of fully 90 per cent of all existing plants. . . . Under normal conditions, and in the long run, one pair of moths, producing say 500 eggs, are represented next year by another pair of the same species, and no more: that is, out of 500 eggs, producing 500 caterpillars, 498 are destroyed in some way. . . . The important thing is that a species abundant in number of specimens has become so in spite of the combination of all its natural checks and, conditions remaining equal, will maintain itself in the same ratio just as a rare species barely maintains itself against the combination opposing it.11

"It has been estimated that there are annually on our coast from New Jersey to Mononomy a thousand million blue fish averaging five or six pounds each. They eat or destroy ten fish a day or a total of ten billion fish a day— weight of 300,000,000 pounds. This takes no account of blue fish under three pounds of weight." 12 If this is a fair estimate of a single variety in a given area, who can visualize the total food consumption of all forms of life on earth? Recognizing that from the egg stage to death, and by no means stopping there, every organism is preyed on by some, not by all others about all we can say is that the survivors are those who have not yet been eaten.

Checks to Increase

There is, however, another check to the increase of organisms even where the food supply itself may be adequate. Changes, especially sudden changes, in the physical conditions destroy countless organisms. In the Chicago drainage canal just below the city, 1,200,000 bacteria per c.c. have been found, but 150 miles away there were only 3,700. While

some of this decrease was doubtless due to the attacks of other forms of life, much was due to the effects of the exposure of sunlight. Early and late frosts destroy many insects. Flood and storm take their toll of plant and animal life, even of man himself. In the middle of February, I saw a killdeer, arrived from the south a month ahead of schedule time. Two days later came a big snow which still covers the ground. That bird is on his way to warmer climes or to destruction ere this.

A change in climatic conditions, an increase or decrease in the rainfall, a warmer or colder season may stimulate the

FIG. 3. FOOD RELATIONS OF AQUATIC ANIMALS

growth of given forms of life which serve as food to others and bring about a disproportionate increase, or scarcity may force sudden migrations of free moving animals. Hence we hear of the enormous flight of locusts, as in Natal in 1893 when one swarm took twenty-one days to pass a given point, or in Mexico in 1924 where they ate every living crop on countless acres. The mice epidemics in Nevada in 1907 and in California during the winter of 1926-1927 will be long remembered. Such an increase inevitably attracts the enemies of the increased species until an equilibrium is again attained. "In this way it happens that after a season of grass

hopper abundance a season of blister-beetle abundance is almost certain to follow, and any abnormal increase of the former is almost sure to be checked by the corresponding increase of the latter." 13

This interdependence of the various forms of life in any given area is well shown by the diagram in Figure 3.

Let us assume that because of some unfavorable conditions in a pond during their breeding season the black bass decrease markedly. The pickerel, which devour young bass must now feed more extensively upon insects. The decreased number of black bass would relieve the drain upon the crayfishes, which are eaten by bass; crayfishes would accordingly increase and prey more heavily upon the aquatic insects. The combined attacks of pickerel and crayfishes would cause insects to decrease and the number of pickerel would fall away because of the decreased food supply. Meantime the bullheads, which are general feeders and which devour aquatic insects, might feed more extensively upon mollusks because of the decrease of the former, but would probably decrease also because of the falling off of their main article of diet. We may thus reasonably assume that the black bass would recover its number because of the decrease of pickerel and bullheads, the enemies of its young.14

The increase of any form of life may depend not only on the presence of some other type of life but upon different types at different stages of its existence. This is true of the fluke producing the liver rot of sheep:

Each fluke produces half a million or more eggs which pass out of the sheep and fall to the ground. The rain washes them into pools and ponds where they hatch, giving forth an active conical creature, exceedingly small, which swims about until it finds a snail, into which it bores its way. Failing the discovery of a snail it perishes; but unfortunately nature provides plenty of snails for it. Within the snail it grows into a sort of sac which in its interior develops another type of young; within these last more young develop, some like the parent, and some like minute pollywogs which emerge from the snail and swim away, climb up a grass blade, lose their tails, and become inert. If

a sheep eats grass with these things clinging to it they come to life again, plod their way into the sheep's liver, and soon develop into full grown liver flukes.15

Comparable phenomena are very common, such as the white-pine blister rust which must pass one stage of its career on plants like the barberry, or the germ producing malaria in man which must live for a time in a mosquito of certain species.

The ichneumon fly (Pimpla) is parasitic on such moths as the army and orchard worms and the white-marked tussock moth, a great enemy of shade trees. It has its own parasites also.

When the tussock moth caterpillars appear in great numbers on our shade trees during a given season, it will almost invariably be found that this Pimpla is present also in great numbers and that the majority of the caterpillars are stung by it. This means that the following season there will be an unusual number of adults of the parasitic insect which is now termed the primary parasite. So great is this abundance that the first generation of tussock moth caterpillars is practically wiped out of existence. Then comes the second curious fact, that the secondary parasites become enormously abundant and kill off the abundant Pimplas. At the end of the same season or at the beginning of the next the tertiary parasites put in their appearance and the secondary parasites are destroyed, thus giving relief again to the primary parasites which once more begin to be abundant and ready for the next case of super-abundance of the host caterpillar.18

Food Habits

A summary of the food habits of organisms may be attempted. Plants feed chiefly on inorganic substances, many of which must have been prepared by bacteria. Plants are sometimes parasitic on other plants as well as on animals, as the mistletoe on the oak or the disease thrush on men. A few plants capture and devour insects. Insects are the commonest of all animal forms, their species far outnumbering