There came a time, maybe 20,000 years ago, when man, instead of (1)______ merely a hunter, started to domesticate animals. The dog helped in his hunting activities, geese and ducks were kept and bred as a source of food, (2)______ was easier than (3)______ to go out and hunt them. Once humans had domesticated animals, they turned their (4)______ to plants. Instead of being nomads, drifting from place to place following the game animals, they began to create farms and thus enter (5)______ more settled way of life. Villages and towns sprang up in places where previously (6)______ had been only a hamlet. Now animals and plants began to be kept not merely for food, but also for interest’s sake or for their beauty. The first writers on animal life was Aristotle, in 33 BC, and Pliny, in 75 AD, but for many hundreds of years after Pliny the subjects of natural history, in common with many (7)______areas of knowledge, hardly progressed at all. For the most part such zoos as existed were in the hands of the dilettante nobility and were no more than second-rate menageries, without (8)______ scientific purpose. However, in the seventeenth century, naturalists began to realize that they needed a system (9)______ classifying living things because as more plants and animals were discovered, it was difficult to (10)______ track of them all.