title: How to force older packages to install on newer versions of R?
body: <p>I cannot install <code>proj4string</code> into my current version of R (2.15.1):</p>
  
  <pre><code>Warning message:
  package ‘proj4string’ is not available (for R version 2.15.1) 
  </code></pre>
  
  <p>I assume this is because 2.15.1 is fairly new and the package just hasn't been updated.  Is there a general mechanism for forcing R to install an older version of a binary package (realizing that there may be bugs that result)?</p>