The endurance of each species and group of species is continuous in time; for the apparent exceptions to the rule are so few that they may fairly be attributed to our not having as yet discovered in an intermediate deposit certain forms which are absent in it, but which occur above and below:
ez ditugula aurkitu oraindik, bitarteko metaketaren batean, bai goitik eta bai behetik falta zaizkion zenbait forma; eta berbera dugu, ziur, arau orokorra espazioan ere, hots, espezie bakar bat edo espezie talde bat bizi den eremua jarraikia dela, eta gertatzen diren salbuespenak, ez hain bakanak, honako arrazoi hauengatik azal daitezkeela, frogatzen saiatu naizenez: