Small animals suitable for experimental use must reproduce fast and their genes should be as similar to humans as possible. Some small animals reproduce fast but have few genes similar to humans. 
some fast-breeding small animals are not suitable for experiments.
To take the course The Philosophy of Cognitive Science, you must have taken Theory of Knowledge and be interested in cognitive science. Some students are very interested in cognitive science, but have not taken Theory of Knowledge . Therefore, some students who are interested in cognitive science cannot take the course The Philosophy of Cognitive Science.