The existence of closely allied representative species in any two areas, implies, on the theory of descent with modification, that the same parent-forms formerly inhabited both areas; and we almost invariably find that wherever many closely allied species inhabit two areas, some identical species are still common to both.
Zeinahi bi eskualdetan espezie hurbil-hurbilak edo errepresentagarriak izateak, eraldaketadun ondorengotzaren teoriaren arabera, herrialde bietan lehenago guraso-forma berek izan zirela inplikatzen du; eta ia hutsik gabe aurkituko dugu, herrialde bitan espezie hurbil-hurbil asko bizi diren guztian, bietako espezie batzuk oraindik berberak direla.