V 
NATURAL SELECTION 
125 
new conditions. And if we remember that all such physical 
changes are slow and gradual in their operation, we shall see 
that the amount of variation which we know occurs in every 
new generation will be quite sufficient to enable modification 
and adaptation to go on at the same rate. Mr. Darwin 
was rather inclined to exaggerate the necessary slowness of 
the action of natural selection ; but with the knowledge we 
now possess of the great amount and range of individual 
variation, there seems no difficulty in an amount of change, 
quite equivalent to that which usually distinguishes allied 
species, sometimes taking place in less than a century, should 
any rapid change of conditions necessitate an equally rapid 
adaptation. This may often have occurred, either to im¬ 
migrants into a new land, or to residents whose country has 
been cut off by subsidence from a larger and more varied 
area over which they had formerly roamed. When no change 
of conditions occurs, species may remain unchanged for very long 
periods, and thus produce that appearance of stability of species 
which is even now often adduced as an argument against 
evolution by natural selection, but which is really quite in 
harmony with it. 
On the principles, and by the light of the facts, now briefly 
summarised, we have been aide, in the present chapter, to 
indicate how natural selection acts, how divergence of char¬ 
acter is set up, how adaptation to conditions at various periods 
of life has been effected, how it is that low forms of life 
continue to exist, what kind of circumstances are most 
favourable to the formation of new species, and, lastly, to 
what extent the advance of organisation to higher types is 
produced by natural selection. We will now pass on to con¬ 
sider some of the more important objections and difficulties 
which have been advanced by eminent naturalists. 
