Darwin believed that there was natural variation, though his theory came before Mendel's work showed just how that variation was encoded genetically, which was heritability. In his view, natural selection worked to allow a greater proportional representation in the next generation from those better adapted to conditions. This would then lead to further generations, biological unity resulting from all life evolving in this way from a common origin.
Work Step by Step
The key here is to review the basic principles of evolution as laid down by Darwin and see both the source of diversity (somewhat puzzling for him since Mendel's work was largely unknown until after Darwin died) and how natural selection can lead to all life originating from one common ancestor, providing the source of biological unity.