EDIT

  $form['cdm_dataportal_show_back_to_search_results'] = array(

      '#type' => 'checkbox',

      '#title' => t('Show <em>Back to search results</em> link at the taxon site.'),

      '#default_value' => variable_get('cdm_dataportal_show_back_to_search_results', 1),

      '#description' => t('<p>If checked the link to search results is rendered at

       the top of the taxon site. Clicking on the link the last search performed

       is rendered again.</p>'),

  );