The Significance of Reproduction
The individual is the triumphant outcome of the age-long interaction of all the mighty and intricate forces of evolution.
The various mechanisms of metabolism thus far considered, including the integumentary, digestive, circulatory, respiratory, and excretory systems, as well as the mechanisms of motion and sensation which are to be described in Part III, all contribute directly to the development and maintenance of the individual. There is indeed only one other concern in nature of greater moment than the up-keep and well-being of the individual and that is the continuation on the face of the earth of those precious products of evolutionary travail which have “won a place in the sun.” Life must go on. Such is nature’s ultimatum, although no single individual, even of the genus Homo, is of so supreme importance that it cannot be spared. Individuals die only to be replaced by others, and in the long run this is a fortunate provision, since it is the fate of every organism, like any other delicate piece of machinery, to wear out and become useless eventually in the natural course of events.
To provide for the life of the species by replacement is the part of reproduction.
Filling the gaps caused by the death of individuals in the rank and file of organisms may be regarded as a matter of extra growth beyond individual requirements at the expense of non-living materials. The surplus thus gained may become detached from the original organism to form a new and independent individual. Excess growth material endowed with the capacity to reconstruct a new individual is termed germplasm, while the body from which it is detached is composed of somatoplasm.
Organisms generally, which are made up of these two kinds of materials, consequently lead a dual life. The mortal somatoplasm is inevitably headed towards eventual death, while the potentially immortal germplasm has opened up before it the possibility of escape from death through reincarnation in a new individual life.
The germplasm thus forms a continuum which joins generations together. Any particular organism represents not something new, like a manufactured article, but the ultimate link in an unbroken chain extending back into the evolutionary past farther than the imagination can follow it. In this way the torch of life is not extinguished but is handed on.
Maintenance of the species, as contrasted with maintenance of the individual, is an unselfish altruistic function, frequently accomplished at the expense of individual comfort, or even at the sacrifice of the individual life.
The effective operation of the function of reproduction on the part of animals lacking the ability to reason and uncontrolled by altruistic motives is insured by being grounded firmly in fundamental urges and universal desires which carry the reward of selfish satisfaction while at the same time accomplishing the altruistic end of providing for others.
That flower of creation, moral man, in spite of the fact that he is by no means entirely emancipated from the effective laws of the jungle, has in comparatively recent evolutionary times set up certain ethical rules to govern the operation of the indispensable function of reproduction that are somewhat at variance with a life of selfishness. Moral responsibility does not worry animals.
It is perhaps biologically fortunate that man, although subjected to an overlay of social restraints, is still bombarded by the same universal compelling physiological urges with their rewards of selfish gratification which serve to safeguard and insure the altruistic and sacrificial ends that result in the perpetuation of the species.
Methods of Reproduction
Sex, in spite of the popular impression to the contrary, is not essential to reproduction. Many organisms reproduce their kind asexually by processes of unequal fragmentation, such as budding and spore formation, or by more or less equal division into two, as in the fission of many protozoans.
The first event in the life of any vertebrate, however, is the union of two highly endowed cells, called gametes, furnished by two different individuals, male and female. The undifferentiated cell thus formed is a “fertilized egg,” or zygote, than which no other cell has so great expectations.
This is sexual reproduction and in describing the mechanism involved one should first of all distinguish the essentially “immortal germplasm” (Weismann) that bridges the generations, from the accessory structures which minister thereto, but are destined to perish with the life of the individual of which they form a part.