question reg. natbib

Dear all,

I am usually referencing like this:

This is a test~\cite{234}.

Where 234 is a reference to a reference within the bib.bib file:

@BOOK{234,
author = "C. R. Darwin",
year = "1860",
title = "On the origin of species by means of natural selection",
publisher = "Murray, London"
}

Now I have to use natbib for referencing. My tex file looks like this:

\usepackage[authoryear]{natbib}
\bibliographystyle{plainnat}
\bibpunct{(}{)}{,}{a}{}{;}

\begin{document}

Bla bla … This is a test~\citep{234}. Bla bla …

\bibliography{bib}

\end{document}

Everything looks fine apart from the actual references at the end of the 
document. The year of publication is not behind the author(s) but 
instead at the end of a reference. Do I have to transform my original 
bib.bib file into bibitems that have to be inserted at the end of the 
document? Using something like this:

\begin{thebibliography}

\bibitem …

\bibitem …

\bibitem …

\end{thebibliography}

Any pointers would be very much appreciated. Many thanks in advance.

Regards,
Christian