The Evolution Of Infancy And Parenthood
( Originally Published 1923 )
The Evolution of Infancy and Parenthood.—The dependence of the new individual carries with it the very elaborate development of special organs and special instincts on the part of the parent. These have to do with the early period of the individual's life, and with the mutual relations between parent and offspring.
Every degree of this dependence may be found among back-boned animals. Among fishes generally, the females lay their eggs in the water and the males pour their sperms-the milt--over the eggs, which are then fertilized and are beyond the further concern of the parents. In relatively few species of fish the parents prepare some sort of rude nest and guard the eggs after they are laid. But even without the instinct for nest building or for guarding the eggs, the adults, that is, the parents, in many species instinctively make further provision for the off-spring by traveling great distances to relatively sheltered places before depositing the roe and milt. This is strikingly illustrated by the Columbia River salmon, which spend most of their life in the ocean, but swim up to the head-waters of streams, jumping rapids and even respectable falls, leave the reproductive cells in shallow pools, and finally, having completely exhausted them-selves, drift down the stream to death.
Among the batrachians or amphibians, fertilization is also generally accomplished outside the mother's body. The adults, although they are thoroughgoing air breathers and land animals, return during the mating season to the water. Here the female frog, for example, passes the eggs out of her body, while the male, sitting on her back and clasping her tightly, discharges the sperm-bearing fluid. The sperm cells, swimming freely in the water, find and fertilize the egg cells as they emerge. Among most frogs, toads, and salamanders the eggs are left to themselves. In a few species of toads there is further protection of the young while they are hatching. In one species the male carries about a mouthful of the developing eggs until the tadpoles are able to swim freely. In another species the female carries a mass of fertilized eggs about on her back; each egg sinks into a little pit in the skin which is formed by the presence of the egg, and there they receive some protection until the young are ready to emerge.
Among the other classes of backboned animals, namely reptiles, birds, and mammals, fertilization always takes place within the mother's body. This involves more elaborate organs for the discharge of the gametes, for the insertion of the sperm into special receptacles within the body of the female, and for the disposal of the fertilized egg.
Among the birds and reptiles the fertilized egg, while still in the mother's body, becomes covered with a mass of food material and a shell, which protects it against loss of water as well as against external injury. This egg deposited by the mother (which is familiar to us in the case of the domestic fowl), is therefore a great deal more than a gamete, more even than a fertilized egg cell. Indeed, by the time the hen lays her egg, the single cell has already undergone several successive divisions, and the "germ spot" seen on top of the yolk consists of eight or sixteen cells, or sometimes more. The yolk and the white are masses of highly concentrated food and water. Upon this material the developing embryo subsists until the young animal comes out of the shell and is able to take food and breathe air. The eggs of snakes, alligators, and turtles are generally left to themselves, being hatched by the heat of the sun. Some species of reptiles keep close to their eggs and off-spring, but generally speaking there is a marked contrast between the reptiles and birds with respect to the relation between parents and offspring.
Among the birds generally the mother supplies not only a large mass of food for the offspring, but body heat during the period of incubation, or hatching. And in very many species, the mother continues to feed the helpless young for a considerable time after hatching. The building of nests, the prolonged "sitting," the gathering of food, and the placing of it in the mouths of the young, the active protection of eggs and young against enemies, and the "teaching" of the young to fly, all represent a very complex set of structures and instincts, or behavior-patterns, that on the whole make for a better start in life on the part of the young.
Among the mammals this elaboration has proceeded still farther. Here the eggs are not only fertilized within the parent's body, but the whole incubation period is passed within the mother's body. Early in the course of its development the mammalian embryo already has its own heart and blood vessels. The food and oxygen from the mother's blood diffuse by way of the lymph through the delicate network of very fine blood vessels in the placenta, the special absorbing organ whereby the embryo attaches itself to the lining of the uterus or womb. This transfer of material from mother to child is accompanied by a transfer in the opposite direction of wastes from the activities of the embryo. The mother thus furnishes the young animal with excretory service as well as with nutritive and respiratory. The uniform temperature essential to the embryo's development is also furnished by the womb of the mother, as well as protection from many other vicissitudes. After birth, the young animal is capable of breathing and excreting, and of digesting its food; but it is quite helpless so far as the getting of food is concerned. Again the mother's body is drawn upon for feeding, during a period varying with the size and the development of the species. Among the higher forms, the infant may remain helpless for many months. And among the different races of mankind variation in the duration of infancy corresponds pretty closely to the advances in culture and capacity.
In passing from the lowest class of backboned animals (fishes) to the highest class (mammals), we thus see an increasing dependence of the offspring upon the parents for the materials and conditions essential for its development—a start in adjustment to life that is characteristic of infancy. At the same time we find a corresponding increase in those forms of behavior which are characteristic of parenthood.
Side by side with the increased elaboration of organs and activities adapted to supplying materials and services to the offspring, there is a striking reduction in the number of young. While fishes produce thousands of eggs, or even hundreds of thousands each season, and may repeat the process year after year, the birds lay two or a few eggs a season, and the mammalian mother bears one or a few children at a time. More-over, there is a certain relation between the duration of infancy and the length of the reproductive period. In the lower members of the series, where maturity is soon reached, the adult stage, during which reproduction may take place, may last fifteen to twenty times as long as the period of infancy; in the higher members, reproduction is possible during a much shorter portion of the whole life, while a much longer portion is given over to the maturing process on one side, and to the care of offspring on the other.
The evolution of infancy and parenthood observed in the animal series is paralleled in the plant series, as we pass from the lowest to the highest. In plants as well as in animals, the more complex the life which a species lives, the more dependent is each generation upon the services of the preceding generation, that is, the more dependent is the offspring upon its parents. And this is true whether we think of the conditions of development merely in physical terms, or include those distinctive stimulations and suggestions that are characteristic of the higher animals, and especially of mankind. As our civilization becomes more complex, the dependence of the infant upon the guidance and training of parents, as well as upon their nourishment and physical care, becomes greater. At the same time, the adult individual gets an increasing amount of satisfaction from helping and serving the young.
Both childhood and parenthood, in the social sense, have become of greater significance than the other aspects of life. Thus is the family rooted in the very nature of protoplasm, no matter what forms it may take under various conditions of living. And the mutual dependence between youth and maturity traces from the very first differentiations between nutrition and reproduction.