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THE SURVIVAL OF THE FITTEST
Oecology.
When a person becomes ill with an infectious disease the immediate aim
of the patient's private doctor or the fever hospital physician is to give the
patient such treatment as is appropriate to hasten recovery. The medical
officer of health, although, of course, concerned with this immediate aim, has a
further interest which is to prevent the spread of the infection to other persons.
In this he is concerned with the nature of the infecting micro-organism, its
mode of multiplication and the channels through which it is conveyed to susceptible
human beings. This leads in turn to a study of the harmful micro-organisms
as one group of many living things, and to the still wider study of the impact of
all living things on each other and the effects of all the different environments
in which they exist. This study of the rise and fall of all living species and their
relationships to environments is the study of oecology.
In the case of any living organism, oecology is concerned with the supply
of food and materials necessary for survival; the rate at which offspring are
produced ; the other living organisms which can act harmfully ; and the
effects of environments, especially adverse ones.
Starvation.
For example, in India and Pakistan the size of the population is determined
by the quantity of food produced by primitive methods, the number of
persons who die of starvation, and the number of persons who die from disease.
Thus, 500,000 die each year from tuberculosis, 50,000 from smallpox ; in
Bengal alone in 1941 there were nearly 400,000 deaths from malaria, over
100,000 from cholera and dysentery, and in Bengal in 1943, 700,000 persons
died of starvation. A problem in applied oecology is given by Professor
F. M. Burnet1; it is to determine the number of whales which can be killed each
season in the Antarctic without risk of extinction of the commercially important
species. To determine the number a great deal must be known about the
migrations of whales, their rate of reproduction and growth, the amount of food
available, and the enemies which attack them. The food supply is itself a
complex problem; whales feed mostly on sea crabs; these crabs feed on
smaller organisms and so on downwards in size until the smallest of all which
feed on the tiny single-celled plants which grow in the upper layers of the sea.
Every species of insect has its own special requirements for the growth of its
larvae such as the amount of salt in the water, the range of temperature, the
movement of the water, and the amount of sunlight. Every species of living
organism from bacteria to man has in some way or other found means to obtain
sufficient food and to resist the attacks of enemies adequately enough to survive.
When the environment changes, as it does constantly, the organisms also
change at a rate near enough to maintain a reasonable equilibrium. They do
so by mutation, that is by slight changes in some of the newborn which enable
them to withstand adverse circumstances. As the number of individuals born
always exceeds the number which can survive, those who have undergone
mutation become the ones most fit to survive whilst the unfit perish.
This process can be watched in any bacteriological laboratory. When food
supply is adequate and other conditions favourable, a colony of bacteria will
1 Burnet, F. M. (1951). Modern Practice in Infectious Fevers, edited by H. Stanley Banks.
Butterworth.
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