Zoologist: Every domesticated large mammal species now in existence was domesticated thousands of years ago. Since those days, people undoubtedly tried innumerable times to domesticate each of the wild large mammal species that seemed worth domesticating. Clearly, ttherefore, most wild large mammal species in existence today either would be difficult to domesticate or would not be worth domesticating.
The zoologist's argument requires the assumption that in spite of the difficulties encountered, at one time or another people have tried to domesticate each wild large mammal species.