I can’t tell without more context what “the material” is. “Yonder” means “over there” or “in the distance”. Nowadays in the US, “yonder” is a word associated with rural speakers (e.g., a farmer saying “My barn is over yonder”). It is not a “city” word unless the city-dweller is consciously trying to sound rural.

Sometimes, in science, "material" can mean "subject matter". However, that doesn't seem to be the way it is used here. Here, it seems to mean that he uses physical things (animals) to answer intellectual questions. After he has performed the experiment, he is left with more physical material--the altered animals. But he only cares about them while he is making them, not after he has made them. He is interested in the experiments and procedures that create them, but the finished creatures are just a byproduct of his experiment. If a chemist is interested in chemical A, and creates chemical B as a byproduct, he will store chemical B in a storage tank until he can throw it away. Moreau is interested in studying plasticity, and the animals are a byproduct: they drip into their huts just like a byproduct chemical would drip into a storage tank, and he stops caring about them after he has made them.