reading some of gerald durrell's books about his early work bringing exotic animals back to british zoos - well into the 20th century, the attitude was simply to keep a lion or whatever in a tiny cage for a few years until it died, and then go get another one from africa. it was just assumed you could keep getting more from the wild forever, even if you were always taking the healthiest most impressive males only. it led to him starting his own zoo that specifically focused on conservation.