The post script was in reference to criticisms in the German Edition to "Origin of Species" specifically about plants. And the holograph leads some to conclude that it was written at a later date, or by someone else. I think the context is that Darwin wrote it. I cannot get the holographic image to copy, which is very frustrating. The post scripted text as transcribed reads:
"In fact the belief in natural selection must at present be grounded entirely on general considerations. (1) on its being a vera causa, from the struggle for existence; & the certain geological fact that species do somehow change (2) from the analogy of change under domestication by man's selection. (3) & chiefly from this view connecting under an intelligible point of view a host of facts.—
When we descend to details, we can prove that no one species has changed: nor can we prove that the supposed changes are beneficial which is the groundwork of the theory. Nor can we explain why some species have changed & others have not. The latter case seems to me hardly more difficult to understand precisely & in detail than the former case of supposed change. Bronn may ask in vain the old creationist school & the new school why one mouse has longer ears than another mouse—& one plant more pointed leaves than another plant."
You will see that the creationist version did not follow the letter text. The section in brackets "[i.e. we cannot prove that a single species has changed];" in the creationist version is an inserted text not in the original letter.
Finally, what Darwin knew in 1863 was much less that we know today. We have directly observed the change in species, and why it has been beneficial. For example, Y. E. Stuart, et al, "Rapid evolution of a native species following invasion by a congener" Science 24 October 2014:Vol. 346 no. 6208 pp. 463-466
DOI: 10.1126/science.1257008