Again, both with plants and animals, there is the clearest evidence that a cross between individuals of the same species, which differ to a certain extent, gives vigour and fertility to the offspring; and that close interbreeding continued during several generations between the nearest relations, if these be kept under the same conditions of life, almost always leads to decreased size, weakness, or sterility.
Honez gain, hala landareei nola animaliei dagokienez, froga argi eta garbiak daude honako hau erakusten dutenak, alegia, neurri bateraino ezberdinak diren espezie bereko animalien arteko gurutzaketak sendotasun eta ugalkortasuna ematen diela haien ondorengoei, eta ahaide oso hurbilen gurutzaketak zenbait belaunaldiz, bizi-baldintza beretan edukitzen badira, tamainuz galtzea, ahultasuna edo antzutasuna dakarkiela.