Nor is their rarity surprising, when we remember how large a proportion of the bones of tertiary mammals have been discovered either in caves or in lacustrine deposits; and that not a cave or true lacustrine bed is known belonging to the age of our secondary or palaeozoic formations.
Hondakin horien urritasuna ez da harritzekoa honako hauek gogoratuz gero ere, alegia, tertziarioko ugaztunen makina bat hezur aurkitu direla bai haitzuloetan edo bai metaketa lakutarretan, eta gure aldi sekundario edo paleozoikoko haitzulo ez egiazko metaketa lakutarrik ez dela ezagutzen.