code: def _species_subdir(
          ensembl_release,
          species="homo_sapiens",
          filetype="gtf",
          server=ENSEMBL_FTP_SERVER):
      
      return SPECIES_SUBDIR_TEMPLATE % {
          "release": ensembl_release,
          "filetype": filetype,
          "species": species,
      }
docs: Assume ensembl_release has already been normalize by calling function
      but species might be either a common name or latin name.