Chap. VI.] SUMMARY. 257 



tion 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 Con- 

 ditions of Existence embraced by the Theory of Natural 

 Selection. 



We have in this chapter discussed some of the 

 difficulties and objections which 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 grada- 

 tions, 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 implies the continual supplanting and extinc- 

 tion of preceding and intermediate gradations. Closely 

 allied species, now living on a continuous area, must 

 often have been formed when the area was not con- 

 tinuous, and when the conditions of life did not insen- 

 sibly 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 from reasons 

 assigned, the intermediate variety will usually exist in 

 lesser numbers than the two forms which it connects ; 

 consequently the two latter, during the course of 

 further modification, from existing in greater numbers, 

 will have a great advantage over the less numerous 



