Linux : Appending a tar file with xargs

Whereas -c switch of tar command creates an archive, -r is used to append files to a tar files. In the following example, xargs keeps on sending htm files as input to the tar command. These files are appended to htm_files.tar.

find . -name "*.htm" | xargs tar rvf htm_files.tar

Usual way of tar -cvf does not work for the above purpose.