154 SUMMARY. [CHAP. Vt 



Summary : the Law of Unity of Type and of the Conditions 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 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 implies the continual supplanting 

 and extinction of preceding 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 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 intermediate variety, and will thus generally succeed in 

 supplanting and exterminating it. 



We have seen in this chapter liow cautious we should be in con- 

 cluding that the most different habits of life could not graduate 

 into each other ; that a bat, for instance, could not have been 

 formed by natural selection from an animal which at first only 

 glided through the air. 



We have seen that a species under new conditions of life may 

 change its habits ; or it may have diversified habits, with some 

 very unlike those of its nearest congeners. Hence we can under- 

 stand, bearing in mind that each organic being is trying to live 

 wherever it can live, how it has arisen that there are upland geese 

 with webbed feet, ground woodpeckers, diving thrushes, and 

 petrels with the habits of auks. 



Although the belief that an organ so perfect as the eye could 

 have been formed by natural selection, is enough to stagger any 

 one ; yet in the case of any organ, if we know of a long series of 

 gradations in complexity, each good for its possessor, then, under 

 changing conditions of life, there is no logical impossibility in the 

 acquirement of any conceivable degree of perfection through 

 natural selection. In the cases in which we know of no inter- 

 mediate or transitional states, we should be extremely cautious in 





