Q:

for as - meaning

An excerpt from Pi: A Biography of the World's Most Mysterious Number:

Since Euler is the father of the symbol that has the title role of this book, we ought to take a glimpse into his interesting life history.  Born in Basel, Switzerland, in 1707, he was initially taught mathematics by this father, who himself studied under the famous mathematician Jakob Bernoulli.  This connection served him well, for as the father noticed his son's proclivity for the subject, he arranged for him to study with Jakob Bernoulli's son (also a famous mathematician) Johann Bernoulli. 

How to understand the construction for as? 

A:

For and as in this sentence are two separate components.  
For can be used as a conjunction meaning "because."  So you could rewrite this as follows:

This connection served him well, because as the father noticed his
  son's proclivity for the subject, he arranged...

"as" here describes a process - the father noticed more and more his son's proclivity, which led him to arrange for him to study with the mathematician. 