How did Charles Darwin view his own findings on evolution to be, hypothesis or fact?

Though, till this day,  scientists debate and differ over whether Darwin’s Theory of Evolution was a hypothesis or fact, Darwin, during his lifetime, expressed his view clearly.

In May of 1866, in an interview conducted by the renowned German botanist Robert Caspary, this exact question was addressed, i.e.  whether Darwin thought that his evolution theory was a fact or a hypothesis.

Caspary reported: “It was important for me to hear, from his own mouth, if he considered his doctrine of evolution of species to be a hypothesis or fact. I asked him if he considered that he had ever found a species anywhere for which it could be established through facts that it was derived from another by changes. – No! he answered very definitively. – Thus you yourself consider that your doctrine of evolution of species is a hypothesis. – O yes! Was his decisive answer.”(1)

Nevertheless, despite Darwin’s personal position, the debate between scientists continues. For example, though totally unjustified, the National Academy of Science responded to the same question in their published booklet on evolution, their answer is: 

“Scientific theories are explanations of natural phenomena built up logically from testable observations and hypotheses. Biological evolution is the best scientific explanation we have for the enormous range of observations about the living world. 

Scientists most often use the word “fact” to describe an observation. But scientists can also use fact to mean something that has been tested or observed so many times that there is no longer a compelling reason to keep testing or looking for examples. The occurrence of evolution in this sense is a fact. Scientists no longer question whether descent with modification occurred because the evidence supporting the idea is so strong.”(2)