Whether or not a
particular mutation will increase the chance
of its possessor to survive and reproduce is dependent upon a second
chain of events, which is quite independent
of the
event of mutation itself.
What in an older kind
of philosophy would have been called the
chain -
of - cause - and - effect is here seen as being very much richer; it is a congeries
of occasions,
events, pressures, movements, routes, which come to focus at this or that point, and which for their explanation require some principle that has brought and still is bringing each
of them, rather than some other possible occurrence, into this
particular concrete moment
of what we commonly style «existence».