We group artworks with similar characteristics into periods or styles because it is easiest to do it that way - this way we can find common characteristics in these literary/artistic eras, and name them. For example, the Renaissance had a distinct style, which differs greatly from that of Romanticism.
Answer:
Black fur is more fit to survive in the environment than other colors of fur.
Explanation:
"Fitness", in terms of natural selection, is an organism's likelihood to survive in a given environment. Assuming that the increase in black fur is significant (Not attributable to pure chance), we can safely assume this is because Black Fur provides a competitive advantage in said environment over other colors of fur. There may be black terrain that allows them to blend into the environment and ovoid predators, or maybe it keeps them warmer if it is located in a cold region, thereby reducing the amount of resources they need to spend to stay warm.
