“On” has become “opposed to;” it is not easy to see why this alteration was made, unless because “opposed to” is longer.
Again:—
“Turning to geographical distribution, the difficulties encountered on the theory of descent with modification are grave enough.”
“Grave” has become “serious,” but there is no other change (p. 461).
Again:—
“As on the theory of natural selection an interminable number of intermediate forms must have existed,” &c.
“On” has become “according to”—which is certainly longer, but does not appear to possess any other advantage over “on.” It is not easy to understand why Mr. Darwin should have strained at such a gnat as “on,” though feeling no discomfort in such an expression as “an interminable number.”
Again:—
“This is the most forcible of the many objections which may be urged against my theory . . . For certainly, on my theory,” &c. (p. 463).
The “my” in each case became “the” in 1869.