Query: What is Darwin's belief about the accumulation of adaptive variations?
Context: He knew that his readers were already familiar with the concept of transmutation of species from Vestiges, and his introduction ridicules that work as failing to provide a viable mechanism. Therefore, the first four chapters lay out his case that selection in nature, caused by the struggle for existence, is analogous to the selection of variations under domestication, and that the accumulation of adaptive variations provides a scientifically testable mechanism for evolutionary speciation.