DIFFICULTIES OF THE THEORY 283 



ration of dense clouds of pollen by our fir trees, so that a 

 few granules may be wafted by chance on to the ovules? 



Summary: the Law of Unity of Type and of the Conditions 

 of Existence embraced hy the Theory of Natural Selection 



We have in tins chapter discussed some of the diffi- 

 culties and objections w^hich may be urged against the 

 theory. Many of them are serious; but I think that in 

 the discussion light has been thrown on several facts, 

 which on the belief of independent acts of creation are 

 utterly obscure. We have seen that species at any one 

 period are not indefinitely variable, and are not linked 

 together by a multitude of intermediate gradations, partly 

 because the process of natural selection is always very 

 slow, and at any one time acts only on a few forms; and 

 partly because the very process of natural selection im- 

 plies the continual supplanting and extinction of preced- 

 ing and intermediate gradations. Closely allied species, 

 now living on a continuous area, must often have been 

 formed when the area was not continuous, and when the 

 conditions of life did not insensibly graduate away from 

 one part to another. When two varieties are formed in 

 two districts of a continuous area, an intermediate variety 

 will often be formed, fitted for an intermediate zone; but 

 I'rom reasons assigned, the intermediate variety will usu- 

 dly exist in lesser numbers than the two forms which it 

 ionnects; consequently the two latter, during the course 

 )f further modification, from existing in greater numbers, 

 vill have a great advantage over the less numerous in- 

 ermediate variety, and wall thus generally succeed in 

 upplanting and exterminating it. 



We have seen in this chapter how cautious we should 



