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 able, 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. 



