We are also led to this same conclusion by considering reciprocal crosses, in which the male of one species cannot be united, or can be united with great difficulty, with the female of a second species, while the converse cross can be effected with perfect facility.
Ondorio berera garamatza elkarrekiko gurutzaketak kontuan hartzeak ere, hauetan espezie bateko arra beste espezie bateko emearekin batzerik ez dagoelarik, eta baldin badago ere, oso zail, alderantzizko gurutzaketa, ostera, erraztasun osoz egin ahal den artean.