THE STRUGGLE FOR EXISTENCE, ADAPTATION, AND SPECIES-FORMING
Technical Note.—Multiplication, or increase by geometric ratio, among animals can be illustrated by noting the many eggs laid by a single female moth or beetle or fly or mosquito or any other common insect (or almost any other non-mammalian animal). The production of many live young by each female rose aphid can be readily seen; the number of young in a litter of kittens or pups or rabbits is a good illustration. From this geometric increase it is obvious that there must be a great crowding of animals and a struggle among them for existence. This struggle and the downfall of the many and success of the victorious few can be observed by rearing in a small jar of water all the young of a single brood of water-tigers (larva of Dyticus) or other aquatic predaceous insect. The strongest young will live by killing and eating the weaker of their own kind. In a spider's egg-sac the young after hatching do not immediately leave the sac, but remain in it for several days. During this time they live on each other, the strongest feeding on the weaker. Thus out of many spiderlings hatched in each sac comparatively few issue. This can be readily observed. Open several egg-sacs and count the eggs in them. Let the spiderlings hatch and issue from some other egg-sacs belonging to the same species of spider. The number of issuing spiderlings will always be much less than that of the eggs. The actual working of natural selection and the forming of new species can of course be seen only in results, and not in process. The great variety of adaptation, the fitness of adaptive structures, can be readily illustrated among the commonest animals. Animals showing certain striking and unusual adaptations will perhaps make the matter more obvious. To all teachers will occur numerous opportunities of illustrating, by reference to actual processes or to obvious results, the principles of this chapter.
The multiplication and crowding of animals.—In the reproduction or multiplication of animals the production of young proceeds in geometric ratio, that is, it is truly a multiplication. Any species of animal, if its multiplication proceeded unchecked, would sooner or later be sufficiently numerous to populate exclusively the whole world. The elephant is reckoned the slowest breeder of all known 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 until a hundred years old. Thus after about eight hundred years there would be, if all the individuals lived to their normal age limit, 19,000,000 elephants alive descended from the first pair. A few years more of unchecked multiplication of the elephant and every foot of land on the earth would be covered by them. But the rate of multiplication of other animals varies from a little to very much greater than that of the elephant. It has been shown that at the normal rate in increase in English sparrows, if none were to die save of old age, it would take but twenty years to give one sparrow to every square inch in the State of Indiana. The rate of increase of an animal, each pair producing ten pairs annually and each animal living ten years, is shown in the following table:
| Years. | Pairs produced. | Pairs alive at end of year. |
| 1 | 10 | 11 |
| 2 | 110 | 121 |
| 3 | 1,210 | 1,331 |
| 4 | 13,310 | 14,641 |
| 5 | 146,410 | 161,051 |
| 10 | ...... | 25,937,424,600 |
| 20 | ...... | 700,000,000,000,000,000,000 |
Some animals produce vast numbers of eggs or young; for example, the herring, 20,000; a certain eel, several millions; and the oyster from 500,000 to 16,000,000. Supposing we start with one oyster and let it produce one million of eggs. Let each egg produce an oyster which in turn produces[19] one million of eggs, and let these go on increasing at the same rate. In the second generation there would be one million million of oysters, and in the fourth, i.e. the great great grandchildren of the first oyster, there would be one million million million million of oysters. The shells of these oysters would just about make a mass the size of the earth.
But it is obvious that all the new individuals of any animal produced do not live their normal duration of life. All animals produce far more young than can survive. As a matter of fact, which we may verify by observation, the number of individuals of animals in a state of nature is, in general, about stationary. There are about as many squirrels in the forest one year as another, about as many butterflies in the field, about as many frogs in the pond. Some species increase in numbers, as for example, the rabbit in Australia, which was introduced there in 1860 and in fifteen years had become so abundant as to be a great pest. Other species decrease, as the buffaloes, which once roamed our great plains in enormous herds and are now represented by a total of a few hundred individuals, and the passenger-pigeon, whose migrating flocks ten years ago darkened the air for hours in parts of the Mississippi valley, where now it is a rare bird. But the hand of man is the agent which has helped to increase or to check the multiplication of these animals. In nature such quick changes rarely occur.
The struggle for existence.—The numbers of animals are stationary because of the tremendous mortality occasioned by the constant preying on eggs and young and adults by other animals, because of strenuous and destructive climatic and meteorological conditions, and because there is not space and food for all born, not even, indeed, for all of a single species, let alone all of the hundreds of thousands of species which now inhabit the earth. There is thus constantly going on among animals a fearful struggle for existence. In the case of any individual this struggle is threefold: (1) with the other individuals of his own species for food and space; (2) with the individuals of other species, which prey on him, or serve as his prey, or for food and space; and (3) finally with the conditions of life, as with the cold of winter, the heat of summer, or drouth and flood. Sometimes one of these struggles is the severer, sometimes another. With the communal animals the struggle among individuals is lessened—they help each other; but when the struggle with the conditions of life are easiest, as in the tropics or in the ocean, the struggle among individuals becomes intensified. Each strives to feed itself, to save its own life, to produce and safeguard its young. But in spite of all their efforts only a few individuals out of the hosts produced live to maturity. The great majority are destroyed in the egg or in adolescence.
Variation and natural selection.—What individuals survive of the many which are born? Those best fitted for life; those which are a little stronger, a little swifter, a little hardier, a little less readily perceived by their enemies, than the others. They are the winners in the struggle for existence; they are the survivors. And this survival of the fittest, as it is called, is practically a process of selection by Nature. Nature selects the fittest to live and to perpetuate the species. Their progeny again undergo the struggle and the selecting process, and again the fittest live. And so on until adjustment or harmonizing of animals' bodies and habits with the conditions of life, with their environment, comes to be extremely fine and nearly perfect.
It is evident, of course, that such a natural selection or survival of the fittest and consequent adaptation to environment presupposes differences among the individuals of a species. And this is an observed fact. No two individuals, although of the same brood, are exactly alike at birth; there always exist slight variations in structure and performance of functions. And these slight variations are the differences which determine the fate of the individual. One individual is a little larger or stronger or swifter or hardier than its mates. The existence of this variation we know from our observation of the young kittens or puppies of a brood. So it is with all animals. Thus natural selection depends upon two factors, namely, the excess in the production of new individuals and the consequent struggle for existence among them, and the existence of variations which give certain individuals slight advantages in this struggle.
Adaptation and adjustment to surroundings.—The action of natural selection obviously must, and does, result in a fine adaptation and adjustment of the structure and habits of animals to their surroundings. If a certain species or group of individuals cannot adapt itself to its environment, it will be crowded out by others that can. A slight advantageous variation comes in time by the continuously selective process to be a well-developed adaptation.
The diverse forms and habits possessed by animals are chiefly adaptations to their special conditions of life. The talons and beak of the eagle, the fishing-pouch of the pelican, the piercing chisel-like bill of the woodpecker, and the sensitive probing-bill of the snipe are adaptations connected with the special feeding habits of these birds. The quills of the porcupine, the poison-fangs of the rattlesnake, the sting of the yellow-jacket, and the antlers of the deer are adaptations for self-defence. The fins and gills of fishes, the shovel-like fore feet of the mole, the wings of birds and insects and bats, the toe-pads of the tree-toad, the leaping-legs of the grasshopper, all these are adaptations concerned with the special life-surroundings of these animals.
Adaptations may relate to habits and behavior as well as to structure. Plainly adaptive are such habits as the migration of birds and some other animals, most of the habits connected with food-getting, and especially striking and interesting those connected with the production and care of the young, including nest-making and home-building.
Species-forming.—It is evident that through the cumulative action of natural selection, animals of a structural type considerably (even unlimitedly) different from any original type may in time be produced by the gradual modification of the original type under new conditions. If, for example, a few individuals of a mainland species should come to be thrown as waifs of wave and storm upon an island, and if these should be able to maintain themselves there and produce young, increasing so as to occupy the new territory, there would be produced in time a new type of individual conforming or adapted to the conditions obtaining in the island, these conditions being, of course, almost certainly different from those of the mainland. Thus as an offshoot or derivation from the original type still existing on the mainland we should have the new island-inhabiting type. Now when these island individuals come to differ so much, structurally and physiologically, from the mainland type that they cannot, even if opportunity offers, successfully mate or interbreed with mainland individuals the island type constitutes a new species. That is, our distinction between species rests not only on structural differences, but on the impossibility of interbreeding (at least for the production of fertile young). Such a combination of the action of natural selection and the condition of isolation (as illustrated by the case of island animals), is probably the most potent factor in the production of new species of animals (and plants).
For accounts of the struggle for existence, variations, adaptations, natural selection and species-forming see Darwin's "Origin of Species," Wallace's "Island Life," and Romanes' "Darwin and After Darwin," I.
Artificial selection.—When a selection among the individuals of a species, that is, the choosing and preserving of individuals which show a certain trait or traits and the destroying of those individuals not possessing this trait, is done by man, it is called artificial selection. To artificial selection we chiefly owe all the many races or varieties of our domesticated animals and plants. For example, from the ancestral jungle fowl have been developed by artificial selection (and by cross-breeding) all our kinds of domestic fowl, as Brahmas, black Spanish, bantams, game-cocks, etc.; from the wild rock-dove have been developed our various fancy pigeons, as carriers, pouters, fantails, etc.
For an account of artificial selection see Darwin's "Plants and Animals under Domestication," and Romanes' "Darwin and After Darwin," I.