Endogenous and exogenous repair

All structures are damaged as time passes. If repair is imperfect, then deterioration is as inevitable as damage. Somatic repair is clearly imperfect, since individual senescence and death probably occur in all organisms where an individual can be unequivocally defined. But while individuals die, lines of descent persist, perhaps indefinitely; if this were not so, life itself could not have persisted through geological time. The repair of the germ line must therefore be much more effective than that of the soma. At the same time, the isolate culture of protozoans provides indisputable evidence that the germ line itself may irreversibly decay under certain conditions. To understand the relationship between somatic and germinal aging, and the distinction between mortal and immortal germlines, we must identify the mechanisms of germ-line repair.

Imagine that we are dealing with some human artefact, such as a chair. More precisely, we should speak of a system in which chairs are one component while the craftsman who makes the chairs is the other. The instructions given to the craftsman are that each set of chairs should correspond exactly, so far as that is possible, with an example of the set of chairs that he made previously. To prevent chairs from successive sets deteriorating into a haphazard pile of sticks and boards, two sorts of procedure might be used. In the first place, each chair might be scrutinized for any departure from the master chair, and any discrepancies made good. Secondly, a whole batch of chairs might be constructed, without repair, and each then subjected to some test of function.