Before you start an experiment, all animals are arranged by rarity in a list at the bottom of the screen.

When you select an animal for the experiment, this animal disappears from the bottom list.
The other animals are arranged in two groups.

The first group consists of animals that can be a paired for the experiment with the selected one.
These animals have flask icons.

The second group consists of animals that cannot be used in this experiment. These animals appear in grayscale.