We may not separate terumah from [produce of one] species on behalf of [produce which is] not of that species. If one sepa-rated terumah [thus], his terumah is not terumah. All types of wheat are one [species]; all types of fresh figs, dried figs and pressed figs are one [species], and one may separate terumah from one on behalf of the other.  Wherever there is a Kohen [available] one should separate terumah from the superior [produce], but wherever there is no Kohen [available] one should separate terumah from the longer-lasting [produce]. Rabbi Yehudah says: One should always separate terumah from the su-perior [produce].