NATURAL SELECTION 49 
study of the nature of individual differences and of 
other kinds of variations, as well as of the manner of 
operation of ‘ the strong principle of inheritance,’ has 
confirmed this view as to the method of origin of 
species, or has led to the introduction of modifications. 
Let it be remembered that this suggestion of a 
natural means of modification had, within a few years, 
the effect of convincing practically the whole thinking 
world of the truth of the theory of organic evolution— 
an effect which all the other arguments recited in the 
last chapter were quite unable to produce, so strong 
was the then existing prejudice in favour of the doc- 
trine of special creation. 
The truth of the general principle of the survival of 
the fittest is quite untouched by recent criticism ; 
but a great deal of argument has been expended over 
the questions: (x) how much fitness is sufficient to 
lead to survival, and (2) whether very small advan- 
tages in the way of fitness, even if they lead to the sur- 
vival of the individuals which exhibit them, will be 
followed to an indefinite extent in succeeding genera- 
tions by further improvements in the same direction. 
We shall find that a good deal of evidence has accu- 
mulated tending to show that the second of these 
questions must be answered in the negative, although 
the point is not yet settled to the satisfaction of 
everyone. The remainder of the present chapter will 
be occupied in discussing some of the arguments which 
bear upon this question. 
An important factor which must probably be added 
to the factors of variation, selection, and heredity, in 
4 
