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if we could conceive such a condition, no change would persist. Without selection, there would be no premium placed on adaptive characters, and organisms would persist in every degree of variance with their surroundings. Without some degree of isolation, every change would be lost by cross-breeding with the mass. In a world of varying conditions with varying organisms, it is not conceivable that species should, through all their generations, undergo no change. Nor in the changes of any species is it possible that any one of the factors or conditions named above should be wholly absent. But the effects of each one may show themselves in many different ways, and each may be modified by other facts or conditions. We have compared the history of species to the flow of a river. A single rock may change the course of a stream. In like manner incidental circumstances may determine the evolution of a species. Or using a different metaphor we may compare the course of a species with that of a glacier. The movement of a glacier depends on the law of gravitation “resident" within its molecules. Its course is determined by the topography of its bed. To this bed it is perfectly fitted, but the condition of its surface depends on circumstances related neither to the law of gravitation nor to the form of its bed. A species of animal or plant is well fitted to its conditions in life. This natural selection rigidly enforces; but its surface characters, which are not essential to its life, are determined by other influences, and in this both selection and environment play but a minor part.

All animals feed upon living organisms or upon that which has been living. Hence each animal throughout its life is busy with the destruction of the other organisms or with their removal after death. If these creatures, animals, or plants on which animals feed, are to hold their own, there must be an excess of birth and development to make good the drain upon their numbers. If the plants did not restore their losses the animals that feed on them would perish. In like fashion flesheating animals are dependent on those which feed on plants.

But throughout nature there is a vast excess in the process of reproduction. More plants sprout than could find standing room were all to grow. More seeds are developed than can find place to sprout. More animals are born than can possibly survive. The process of increase among animals is rightly called multiplication. Each species tends to increase in geo

metric ratio, but as it multiplies it finds the world already crowded with other multiplying species. A single pair of any species whatsoever, if not checked by adverse conditions, would soon fill the whole earth with its progeny.

An annual plant producing two seeds only would have 1,048,576 descendants in twenty-one years, if each seed sprouted and matured. But most plants produce hundreds or thousands of seeds. The ratio of increase is a matter of minor importance. It is the ratio of increase above loss which determines the fate of species. Those species increase in numbers in which the gain exceeds the rate of destruction through the influence of other species or the adverse conditions of life. Where few enemies exist the ratio of increase need not be large. One of the most abundant of birds is the fulmar petrel of the midPacific. It lays but one egg yearly, but it has few enemies and the low rate of increase suffices to cover the sea with fulmars within the region it inhabits.

It is not easy to realize the inordinate numbers any species would attain were it not for the checks produced by the presence of the activity of other organisms. Certain protozoa, at their normal rate of increase-if none were devoured or destroyedmight fill the entire ocean within a very short time. It is said that the conger eel lays 15,000,000 eggs yearly. If each hatched and the conger grew to maturity, in a few years there would be no room for any other kind of fish in the sea. The codfish has been known to produce 9,100,000 eggs each year. If each egg were to develop, in ten years the sea would be solidly full of codfish.

The female quinnat salmon of the Columbia, Oncorhynchus tchawytscha, ascends the river at the age of about four years, and lays 4,000 eggs, after which she dies. Half these eggs develop into males. If each female egg came to maturity, we should have at the end of fifty years 8,000,000,000,000,000,000,000,000,000,000,000,000,000,000 female salmon and as many males as the offspring of a single pair. It takes about one hundred of these salmon to weigh a ton. Could all these fishes develop, in a very short time there would be no room for them in all the rivers of the North, nor in all the waters of the sea.

If each egg of the common house fly should develop and each of the larvæ should find the food and temperature it needed, with no loss and no destruction, the people of the city in which it happened would suffocate under the plague of flies. Whenever any species of insect develops a large percentage of the eggs laid, it becomes at once a plague. Thus originate plagues of locusts, grasshoppers, and caterpillars. But the crowd of life renders these plagues rare. Scavenger-beetles and bacteria destroy the decaying flesh where the fly would lay its eggs. Minute creatures, bacteria, protozoa, other insects, are parasitic within the larva itself. Millions of flies starve to death. Millions more are eaten by birds and predaceous insects. The final result is that from year to year the number of flies does not increase. Linnæus once said that “three flies will devour a dead horse as quickly as a lion." Quite as soon would three bacteria with their descendants reach the same result. “Even slow-breeding man,” says Darwin, “has doubled in twenty-five years. At this rate in less than a thousand years there literally would not be standing room for his progeny. The elephant is reckoned the slowest breeder of all animals. It begins breeding when thirty years old and goes on breeding until ninety years old, bringing forth six young in the interval and surviving to be a hundred years old. If this be so, after about 800 years there should be 19,000,000 elephants alive descended from the first pair.” A few years of still further multiplication without check, and every foot of the earth would be covered by elephants.

Similar calculations may be made in regard to any species of animal or plant whatsoever. Each one increases at a rate which without checks would make it soon cover the earth. Yet the number of individuals in a state of nature in any species remains about stationary. With the interference of man, in many species the numbers slowly diminish; very few increase. There are about as many squirrels in the forest one year as another, as many butterflies in the field, as many frogs in the pond. Wolves, bears, deer, ducks, singing birds, fishes, all suffer from man's attacks or man's neglect and grow fewer year by year. It is manifest that the tendency to reproduce by geometric ratio meets everywhere with a corresponding check. This check is known as the Struggle for Existence.

The struggle for existence is threefold: (a) Among individuals of one species, as wolf against wolf or sparrow against sparrow; (b) between individuals of different species, as rabbit with wolf or blue-bird with sparrow; (c) with the conditions in life-as

the necessity of the robin to find water in summer or to keep warm in winter. All three forms of the struggle for existence, intraspecific, interspecific, and environmental, are constantly operative and with every species. In some regions or under some conditions the one phase may be more destructive, in others another. Any one of these may be in various ways modified or ameliorated. When the conditions of life are most easy, as with most species in the tropics, there the conflict of individuals and the conflict of species is most severe. It is not possible to say that any one of these three forms of struggle and selection is more potent than the others. In fact, the first and the second are in a sense forms of the third. All struggle is, strictly speaking, with the conditions of life. Those individuals

Fig. 38.-Praying mantis, eating a grasshopper. (Adapted from photograph from life by Slingerland.)

which endure this struggle survive to reproduce themselves. The rest die and leave no progeny. Because of the destruction resulting from the struggle for existence, more individuals in each species are born than can mature. The majority fail to reach maturity because for one reason or another they cannot do so. All live that can. Each animal tries to feed itself: many try to take care of their young. But in self protection and in propagation of the species very few individuals succeed in comparison with the vast number which the process of reproduction calls into being. The destruction in nature is not indiscriminate. In the long run and for the most part, those creatures least fitted to resist are the first to perish. It is the slowest animal which is soonest overtaken by the pursuers. It is the weakest which is

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crowded aside or trampled on by its associates. It is the least adaptable which suffers most from extremes of heat and cold. By the process of Artificial Selection the breeder improves his stock, destroying his weakest or least comely calves, reserving the strong and fit for parentage. In like fashion, on an inconceivably large scale, the forces of nature are at work modifying and fitting to the demands of their surroundings the different species of animals. Because the processes and results of the struggle for existence seem parallel with those of artificial selection, Darwin suggested the name of Natural Selection for the sifting process as seen in nature. To the general result of natural selection, Herbert Spencer has applied the term

Fig. 39.- The Australian ladybird, Vedalia cardinalis, feeding on cottony cushion scale.

Icerya purchasi. (From life.)

Survival of the Fittest. By fitness in this sense is meant only adaptation to surrounding conditions, for the process of natural selection has no necessary moral element, nor does it necessarily work toward progress among organisms. With changing conditions species undergo change. Some individuals, by the possession of slight advantageous variations of structure or of instinct, meet these new demands better than others. These survive, the others die. The survivors produce young sharing in part, at least, their own advantages, and with renewed selection the degree of adaptation increases with successive generations.

To the process of natural selection we must, in most cases, probably ascribe the adjustment of species to surroundings.

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