This is clearly shown by hybrids never having been raised between species ranked by systematists in distinct families; and on the other hand, by very closely allied species generally uniting with facility.
Hau argi ageri da honako hau ikustean, alegia sailkatzaileek familia ezberdinetan sailkaturiko espezieen artean sekula ez dela hibridorik erdietsi eta espezie hurbil-hurbilak, aitzitik, erraz batzen direla.