We can further understand the singular fact remarked on by several observers that the productions of Europe and America during the later tertiary stages were more closely related to each other than they are at the present time; for during these warmer periods the northern parts of the Old and New Worlds will have been almost continuously united by land, serving as a bridge, since rendered impassable by cold, for the intermigration of their inhabitants.
Halaber uler dezakegu behatzaile ezberdinek seinalatu duten egitate berezi bat, hau da, Europa eta Amerikako ekoizkinak tertziarioko azken estratuetan egun dauden baino elkarrekin erlazionatuago zeudela; izan ere aldi beroago haietan mundu zahar eta berriko iparraldeak baturik egon behar izan zuten etengabeko lehorrez, pasabidea hango biztanleek batetik bestera, norabide bietan, migratzeko zubi izanik, hotzak zeharkaezin bihurtu zuen arte.