variable in filter doesn't work??

when I put a variable in a filter I am having problems...

here is my xml:
-----------------------
<people>
	<person name="george">
		<cat>zoro</cat>
		<dog>butch</dog>
		<fish>jaws</fish>
	</person>
	<person name="jennifer">
		<cat>felix</cat>
		<dog>fido</dog>
		<fish>moby</fish>
	</person>
	<person name="simon">
		<cat>tom</cat>
		<dog>scooby</dog>
		<fish>conroy</fish>
	</person>
</people>
-----------------------

this xsl works properly (it sorts out all except for when cat='zoro':

-----------------------
<xsl:for-each select="//person[cat = 'zoro']">
	<tr>
		<td><xsl:value-of select="cat"/></td>
		<td><xsl:value-of select="dog"/></td>
		<td><xsl:value-of select="fish"/></td>
	</tr>
</xsl:for-each>
-----------------------

output:
zoro   butch   jaws

why doesn't this work?
-----------------------
<xsl:variable name="filter" select='"cat = &apos;zoro&apos;"'/>
<xsl:value-of select="$filter"/>
	<tr>
		<td><xsl:value-of select="cat"/></td>
		<td><xsl:value-of select="dog"/></td>
		<td><xsl:value-of select="fish"/></td>
	</tr>
</xsl:for-each>
-----------------------
output:
zoro   butch   jaws 
felix   fido   moby 
tom   scooby   conroy

I really trying to filter based on a parameter, if the parameter is empty, don't filter.  If it is something, filter.

thanks,
will

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