The hunter

Richard Sinclair worked for a zoo. He was a hunter. When the zoo wanted new animals, Richard went out and caught them. He was a very good hunter and he had an unusual way of catching animals.
One year the zoo needed some African tree monkeys so Richard went to Africa to catch some. He took a new assistant with him. Richard also took a very large dog with him. It was an angry dog and it had very large, sharp teeth. It often bit people so Richard tied a piece of rope around the dog's mouth to keep it closed.
The three of them travelled to Africa. They went into the jungle to an area where the tree monkeys lived. When they saw a tree monkey in a tall tree in front of them, Richard took out a large bag and a gun. He gave them to the assistant.
"What will I do with these?" the assistant asked.
"Listen carefully," Richard said. "I will climb the tree. Then I will shake the tree very hard and a monkey will fall out of it."
The assistant thought for a minute. Then he said,
"But the monkey is very fast and it will run away before I can catch it."
"Don't worry," Richard replied. "Take the rope from the dog's mouth. It is very angry and very fast. It will catch the monkey and bite it with its sharp teeth. The dog will hold the monkey in its mouth. You follow the dog and put the monkey in the bag.
Very easy. Very simple and it works every time. Do you understand what to do?"
The assistant thought for a few seconds. Then he said,
"Yes. I understand what to do. But what do I need the gun for?"
Richard smiled and said,
“If I fall out of the tree, shoot the dog.”