Hi

You can read Darwin's books over the Internet at

Darwin's Origin of Species

but remember that when Darwin was writing scientists did not know 
anything at all about genetics, cell biology, environmental science, 
ecology or microbiology! At that time we were just starting to understand 
how the world works. So naturally you could not expect Darwin to come up 
with complete explanations for every detail of Evolution! :)

The main things that Darwin was trying to explain were:

   -- living things continually change and mutate.

   -- if the changes are useful and help the animal or plant to
      survive, then those species will prosper and spread, but if the
      change is not useful then the descendants of that line will probably
      die out. 

   -- since conditions on the earth change, what is useful at one time may
      not be useful later... the number of different species and the 
      proportion of different species will change over time.

Instead of me trying to explain a lot of the details, you might like to connect to these sites:

 The University of California Evolution site

and

 Human Evolution -- a web page at the University of Indiana

thanks for the question!

matt