Answer:
It is the best understanding that we have of that part of the natural world.
Step-by-step explanation:
Darwin explained the process of evolution driven by the mechanism of natural selection. His idea was based on his observation about similarities and differences in the species present on the Galapagos Islands and the mainland.
He also put forward the analogy in the form of artificial selection, the process of development of desired varieties of crops and breeds of domestic animals.
Since Darwin was able to explain the process of natural selection as a mechanism for evolution scientifically and put forward several pieces of evidence to support it, his ideas about the same are being regarded as a theory.