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of prey had destroyed them all. But would it not have 
been different if the young had not unlearned the “blinding” 
instinct which gave safety to the young of the ancestral 
race ? * 
When it is contended that slight variations might be 
profitable, and therefore become the objects of the action 
of Natural Selection, we should bear in mind that this 
involves the severest stress in the struggle for existence. 
It implies that the few individuals (if any) in each 
generation which happen to vary in their instinctive action 
in a way favourable to the well-being of the race, should 
be preserved while all the rest are destroyed ; and that this 
process should be continued generation after generation. 
No species could survive a treatment like this, even if a 
very small change were favourable. 
It might, however, be contended that it was a mistake 
to insist on the variations of instinct being slight. They 
might be very considerable. But if so, they would be 
proportionately prejudicial so long as circumstances did 
not change ; and would be suppressed by Natural Selec 
tion accordingly, if there is any force in the argument 
that the stability of species is preserved by Natural Selec 
tion. These considerable variations might be an advantage 
so far as they happened to accord with change of external 
conditions. But even in that case the few who possessed 
these advantages would alone survive ; they would at 
once become a rare species. If a second disability were 
met in the same way, they might easily become an extinct 
species. 
When I consider all these difficulties, I come to the 
conclusion that it was a true insight, and not mere 
modesty, which led Mr. Darwin to suppose that the 
* p. 109.