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scale of existence. Nor does he discuss the origin of 
instinct, but only the development of higher instincts from 
lower ones. 
“ I may premise that I have nothing to do with the origin of the 
mental powers, any more than I have with that of life itself. We are 
concerned only with the diversities of instinct and of the other 
mental faculties in animals of the same class.”—(f>. zoj.) 
The problem being thus limited, its solution is attempted 
on the lines of demonstration already employed in the 
transmutation of species. In order that instincts should 
be modified by Natural Selection, the first necessity is that 
instincts should vary. “If it can be shown that instincts 
vary,”* is therefore the first postulate. In the second 
place instincts must vary “ spontaneously.” “ I believe 
that the effects of habit are in many cases of subordinate 
importance to the effects of the Natural Selection of what 
may be called spontaneous variations of instincts ;—that 
is, of variations produced by the same unknown causes 
which produce slight deviation of bodily structure.” f We 
may assume that these spontaneous variations are vari 
ations which arise in connection with sexual reproduction. 
It is only by a happy coincidence that favourable variations 
can arise, and only by a deadly struggle that favourable 
variations can be isolated for breeding purposes by Natural 
Selection. In the case of a perfect instinct adapted to 
existing circumstances, no variations can be favourable so 
long as the conditions remain unaltered. This is implied 
by Mr. Darwin when he says: “ Under changed conditions 
of life it is at least possible that slight modifications of 
instinct might be profitable to a species.”} 
In the next place instincts must vary slightly. “Natura 
non facit saltum is applicable to instincts as well as to 
* p. 206. + p. 206-7. 
+ Origin of Species, p. 206.