There are a lot of Scheme implementations floating around the web, in any language you can imagine. I have one of my own (shameless plug: http://code.google.com/p/bobscheme/) in Python & C++, and there are many others, at least some of which are better than mine in some respects.

The implementation linked here is IMHO badly written - that's not good code to learn from. So if your goal is to learn how to implement Scheme, I suggest to look elsewhere. For example, one good resource is Scheme from Scratch: http://michaux.ca/articles/scheme-from-scratch-introduction

Note that this is nothing personal, I just frankly think this is not good code to learn from.