This latter statement is clearly proved by the difference in the result of reciprocal crosses between the same two species, for, according as the one species or the other is used as the father or the mother, there is generally some difference, and occasionally the widest possible difference, in the facility of effecting an union.
Azken baiezpen hau argi frogaturik uzten du espezie biren arteko elkarrekiko gurutzaketen emaitzean dagoen aldeak; izan ere, gurutzaketan espezie bata edo bestea aita edo ama izanik erabiltzen den arabera, batzeko erraztasunean alderen bat izaten da eskuarki, eta, noizbehinka, litekeen alderik handiena.