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CHAPTER XX

THE LAWS OF POPULATION: THE DEATH RATE

Deaths are Either Natural or Accidental. For the purposes of analysis the death rate should be divided into two parts: the natural death rate, and the casual or accidental death rate. All complex organisms are subject to the great law of disintegration. Each animal species has its own life cycle, and death at the end of that period is inevitable. Such deaths comprise the natural death rate. Death may be encountered, however, from various causes at any period of the life cycle, and such deaths may be called accidental.

The natural death rate, which is theoretically the lower limit of mortality, may be easily estimated provided the life period of a species be known. The life period of man however cannot be stated exactly. Karl Pearson, from his “mortality curve," 1 placed the theoretical limit of life at 106.5 years, though he laid little stress on the exact figures, for the theoretical limit varied appreciably with slight changes in the curve. Certainly enough persons live to be 100 years old to show that such an age is within the limits of possibility for the species, and for purposes of illustration the number is a convenient one. However, it may reasonably be argued that 100 years is an exceptional age, and the great majority of persons who reach senility die at an earlier age, say 80 years. In fact Pearson's mortality curve shows that the largest number of those who reach the period of decline die at the age of 72.

If we assume that the life period of man is 100 years and that everyone lives to be 100 years old, — thus eliminating all acci

dental deaths, – then the death rate in a stationary population would be 10 per 1,000. On the other hand, if we assume that the life period is 80 years, the death rate under the same conditions would be 12.5 per 1,000. In a society in which the population is increasing, however, these rates would be materially lowered, for the number approaching the end of the life period would bear a smaller ratio to the total population. A population increasing at the rate of 20% in 10 years, which has been approximately the rate in the United States the last two or three decades, would have a mortality rate of 5-58, if 80 years be taken as the life period; and a rate of 3.54, if 100 years be assumed to be the life period.

1 The Chances of Death, p. 29.

In the United States at the present time it would be fair to take 5.58 as the natural death rate. The actual death rate in the registration area of the United States in 1917 was 14.2. This leaves a rate of 8.62 per 1,000, theoretically possible of elimination. But if 100 years be accepted as the life period, it would mean that 10.66 deaths per 1,000 would come within the possibilities of elimination. The accidental death rate can of course never be wholly overcome, nevertheless the natural rate gives a theoretical limit towards which society may direct its efforts. It should not be understood from these figures that of the actual death rate of 14.2, 5.58 is natural and 8.62 casual. Practically the whole of the present death rate may be called accidental. Less than a half of one per cent of the total population reaches the age of 80. As the casual rate is lowered, the natural rate will rise until it reaches 5.58, when the casual rate reaches zero. This figure presupposes, however, a rapid rate of increase of the population. As the rate of increase declines, the natural death rate will rise, simply from the fact of the smaller divisor, until it reaches 12.5 in a stationary population.

The elimination of accidental deaths does not mean that everyone must die of old age. Deaths due to diseases may be counted as natural, provided they come at or near the end of the life period. Progress in our control over disease is shown

e

1 The formula, which was given me by Professor William E. Story of Clark University, is as follows: - d

in which d is

(1 + e) "" the death rate, e the rate of increase of the population, and m the assumed length of life.

- I

by the decline in deaths from diseases attacking the young, like tuberculosis; and by the resulting increase in deaths from old age diseases, such as pneumonia and heart disease. We may hope to reduce the total death rate, but not the death rate for each disease equally. The forces of natural selection do not act to eliminate diseases which attack the individual after the reproductive period is passed. Medical skill alone must be depended upon to overcome such diseases, and medical skill is helpless after the organism is weakened by age.

Causes Determining Longevity. Turning now to a more detailed analysis of causes of death we may consider first the natural death rate, or, in other words, the life period.

Many explanations have been given to account for differences in duration of life for different species. One theory is that length of life with animals depends upon size. It is true that a rough relationship seems to exist between size and length of life in many species, but not in all. According to Weismann, the elephant lives 200 years, the walrus 100 years, the horse 40 years, the blackbird 18, the mouse 6, and many insects only two weeks. On the other hand, the pike and the carp live 200 years, the toad and the cat 40 years, certain small anemone 50 years, swine and crabs 20 years."

Others have tried to explain duration of life in terms of the vital processes, making it depend on the rate at which the animal lives. And, although this factor may furnish a useful check on the theory of size, it cannot by itself explain all the phenomena. Amphibia, which are sluggish, live, to be sure, for a comparatively long period; but birds, which are more active and have a higher temperature, are also long lived, usually surviving longer than the majority of amphibians of similar size.

Professor Minot traces length of life to the rate of cell change; but all explanations in terms of physiological changes seem to interpret age too much in its own terms. They give the process rather than the cause of decline. Cell changes as well as age itself require explanation.

1 Weismann, Essay upon Heredity, Vol. I, Ch. 1, p. 6. It seems as though Weismann had got hold of some particularly robust specimens. In Buffon's day these animals were less hardy. He reports the following ages of animals: elephants 100 years, horses 30, carps 150, toads 36, cats and swine 20.

Duration of life probably results from a rather complicated process of adaptation of several individual characteristics to the requirements of the environment. The constitutional characters involved include such traits as size, complexity of structure, and degree of activity. And adaptation to the environment means that the species, as well as the individual, must survive. Individual characteristics set certain limits to duration of life, but within these limits considerable variation is possible according to the needs of the species. Length of life then is probably one of several interrelated factors, being closely connected with the length of the period of growth and with the reproductive faculty.

To make this point clearer it will be helpful to refer again to Spencer's law of population, that generation varies inversely with individuation. Individuation may manifest itself in various ways, one of which would be length of life, and in most animal species length of life and rate of multiplication vary inversely. But such a formula would not fit the facts exactly. Another and more important form of individuation is individual development. If individuals of a species require little development after birth to adapt themselves to the environment, they may reproduce at a comparatively rapid rate and live for a considerable period also. If much individual development is needed, - and this is measured by the period of dependence of the young, - the rate of reproduction and the length of life cannot both be great. Metchnikoff 1 opposes the theory that low fertility always accompanies longevity, citing the cases of ducks, geese, and hens, which often live for many years notwithstanding their high degree of fertility; but in all these cases little development is required for adaptation to the environment. Weismann suggests that in their case length of life has developed to offset the frequent loss of life to which the young of such species are subject. And this is much the same explanation from another point of view. Such animals have developed small means of defence, or, in other words, their individuation is

1 Prolongation of Life, p. 45.

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slight, and they fall easy prey to their enemies. The doubtful point is whether they have developed a long life period because they are preyed upon by other species, or whether they are destroyed so easily by enemies because they are so numerous and have small means of defence.

It is evident that any species must adapt itself to its environment with a reproductive power at least sufficient to perpetuate the species. As the need of parental care increases fewer offspring are produced at a time and the reproductive period is gradually lengthened. For the human species the normal life then should be long enough to cover three periods, the period of infancy or dependence, the period of reproduction, and in addition a period approximating the duration of infancy, to enable parents to care for offspring born near the end of the reproductive period. Among the lower animals the first and third periods are short and in many cases the third is entirely eliminated, but in the human species these two periods are greatly prolonged. Among primitive men the period of infancy ceases at about the beginning of adolescence but among advanced peoples the requirements of education cause it to be extended beyond adolescence. The period of dependence in advanced societies must be considered to last approximately to the age of marriage, which may be roughly placed at twenty-five years (in England it averages 26.4 years for males and 24.8 for females). This leaves about 20 years for the period of reproduction. The third period, of care for late offspring, is rather indefinite in length. But these three periods together should bring the normal length of life up to sixty years at least. That is, individuals with a life period of less than sixty years would in the long run be at a disadvantage in the rearing of offspring. It is interesting to note in this connection that under present conditions the expectation of life for persons who reach maturity,

that is, of persons from 20 to 30 years of age, is that they will survive to an age between 60 and 65 years.

Metchnikoff a believes that old age is brought on by the action of certain unfriendly bacteria, and if this should prove to be the

1 Bailey, Modern Social Conditions, p. 338.
: The Prolongation of Life.

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