Twentieth-century science revealed this principle as a result of various experiments on living things. All their efforts to produce a new species through cross-breeding were pointless, revealing that there are insuperable walls between living species. It was definitely impossible to livestock breeders breeding new variations of cows to turn cows into another species altogether, as Darwin had claimed was possible.
Norman MacBeth revealed the invalidity of Darwinism in his book Darwin Retried:
The heart of the problem is whether living things do indeed vary to an unlimited extent. . . . The species look stable. We have all heard of disappointed breeders who carried their work to a certain point only to see the animals or plants revert to where they had started. Despite strenuous efforts for two or three centuries, it has never been possible to produce a blue rose or a black tulip. 191
191 1 Norman Macbeth, Darwin Retried: An Appeal to Reason, New York: Harvard Common Press, 1971, p. 33