Parametrized xsl:sort

I want to call a template which includes an xsl:sort tag, and send the value
of the xsl:sort select attribute as a parameter to the template.
Something like:

	<xsl:call-template name="petTable">
		<xsl:with-param name="selection" select="pet[@species='dog']"/>
		<xsl:with-param name="sortkey" select="@name"/>
		<xsl:with-param name="sortorder" select="'descending'"/>
	</xsl:call-template>

	<xsl:template name="petTable">
		<xsl:param name="selection" select="*"/>
		<xsl:param name="sortkey" select="''"/>
		<xsl:param name="sortorder" select="''"/>
		...
		<xsl:apply-templates select="$selection">
			<xsl:sort select="$sortkey" order="descending"/>
		</xsl:apply-templates>
	..>
	</xsl:template>

However this does not work. Is there a syntax that lets me do this, or is it
just not possible?
Regards,
Neale

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