Fourthly, how can we account for species, when crossed, being sterile and producing sterile offspring, whereas, when varieties are crossed, their fertility is unimpaired?
Nola azaldu, espezieak gurutzatzen direnean antzuak izatea edo ondorengo antzuak sortzea, aldakiak gurutzatzen direnean, ostera, ugalkortasun paragabea duten artean?