Supplemental Reading - "Random Walks in Biology" by Howard C. Berg
It is a good idea mention a nice place to go for more reading on the issues invoked in Chap. 4 of Nelson (and continuing for the next few chapters). "Random Walks in Biology" by Howard C. Berg is a concise and well written classic. It is easy to read and less than 150 pages in length. Best of all, it is in Dover editions - you can get it on Amazon for about $30 (only $20 for a Kindle edition).