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362 Jean Bourgeois-Pichat
into account the age at which contraceptive methods begin to be used, and
we shall distinguish between sexual intercourse that is sanctioned as opposed
to sexual intercourse that is not. We shall also have to bear in mind the various
contraception techniques, as well as the various ways of effecting sexual
intercourse. No doubt the latter have a certain bearing on the efficacy of con-
traception.
Endogenous and exogenous mortality
29. Let us return to that child born alive, usually well proportioned though
occasionally deformed, whom we left just now at the very moment he was
starting life in this world. At the instant when the fertilized ovule begins its
new existence, an ageing process is set in motion. During a certain time the for-
ces of growth and of decay are going to co-exist in the same individual, then
gradually the latter will gain the upper hand and lead him on finally to his
death. But before this final event takes place, the individual will have to go
through a series of crises characterized by a variety of biological disorders
putting his existence in danger which he will have to overcome.
That, grosso modo, is the sequence of biological phenomena that will accom-
pany him from the cradle to the grave.
30. The causes of biological disorders leading to illness can be classified under
two main headings : either the individual carries the cause in question within
him, or else it is brought to him by the external environment. However, this
distinction between endogenous and exogenous causes of disease is a little too
arbitrary. A disease is always the result of a variety of causes in which endoge-
nous and exogenous are mixed together. It seems that individuals have certain
predispositions for contracting a particular disease. In the case of cancer of the
lung, for instance, it is possible to distinguish between three separate classes of
persons: those who are predisposed to this disease and will contract it even if
they do not smoke; those who are proof against it and will not be affected even
if they do smoke; and those who contract the disease through smoking but who
can avoid it by not smoking, or, at any rate, by smoking less. Similarly, it is
often hard to distinguish between a deformity due to some alteration in the
chromosomes which occurred at the time of conception and one acquired
during life in the uterus, due, for instance, to some disease of the mother. As we
come to understand all these mechanisms better, demography will have to
consider the re-classifying of deleterious factors, and it will no doubt be led to
adopt a more subtle distinction than the one which consists in acknowledging
only the two sets of causes - endogenous and exogenous.
3 I. In an endogenous disease, the deleterious factor can be present at birth in the
form, for example, of a chromosomal alteration, but it can also be acquired in
the course of existence, as a result of the actual functioning of the life-giving
mechanisms. These two possibilities are largely related to two sets of causes of
disease: deformities and senescence.
In the case of an exogenous disease, the ailment springs from an encounter