1. A group of biologists conducted an investigation on a remote archipelago (a collection of islands) in the Atlantic Ocean. They measured the claw length of a large number of mice of a single species living on the islands. The mouse species had no natural predators.

After the biologists' visit to the islands, a species of snake that preyed on the mice was accidentally introduced on half of the islands. These snakes were not able to climb trees but they could move very fast over open ground. Several years after the snakes were introduced the biologists returned and found that the claw length of the mice on the snake infested islands had increased while the claw length of the mice on the islands without snakes remained unchanged. The longer claws enabled the mice to climb trees, while shorter claws enabled them to run faster.

State the purpose of including the islands where no snakes were present in the biologists' investigation.