182 Luck, or Cunning ? 
this when he spoke of “my theory.” ‘My theory” 
became “‘ the theory ” in 1869. 
Again :— 
“On the theory of natural selection we can clearly 
understand the full meaning of that old canon in natural 
history, ‘ Natura non facit saltum.’ This canon, if we look’ 
only to the present inhabitants of the world is not strictly 
correct, but if we include all those of past times, it must 
by my theory be strictly true” (p. 206). 
Here the natural interpretation of ‘by my theory” 
is “by the theory of descent with modification ;” the 
words “on the theory of natural selection,’ with which 
the sentence opens, lead us to suppose that Mr. Darwin 
regarded natural selection and descent as convertible 
terms. ‘“‘ My theory’ was altered to “this theory” in 
1872. Six lines lower down we read, ‘‘ On my theory unity- 
of type is explained by unity of descent.” The “ my ” here 
has been allowed to stand. 
Again :— 
“Again, as in the case of corporeal structure, and 
conformably with my theory, the instinct of each species is 
good for itself, but has never,” &c. (p. 210). 
Who was to see that “my theory” did not include 
descent with modification? The ‘““my” here has been 
allowed to stand. 
Again :— 
“The fact that instincts .. . are liable to make mis- 
takes ;—that no instinct has been produced for the exclusive 
good of other animals, but that each animal takes advantage 
of the instincts of others ;—that the canon of natural 
history, ‘ Natura non facit saltum,’ is applicable to instincts 
as well as to corporeal structure, and is plainly explicable 
on the foregoing views, but is otherwise inexplicable,— 
all tend to corroborate the theory of natural selection” 
(p. 243). 
We feel that it is the theory of evolution, or descent with 
