DIFFICULTIES ON THEORY 183 



thrown on several facts, which on the theory of inde- 

 pendent 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 will always be very slow, and will act, 

 at any one time, only on a very few forms ; and partly 

 because the very process of natural selection almost 

 implies the continual supplanting and extinction of pre- 

 ceding 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 in- 

 termediate variety will often be formed, fitted for an 

 intermediate zone ; but from reasons assigned, the inter- 

 mediate 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 how cautious we should 

 be in concluding 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 selec- 

 tion from an animal which at first could only glide 

 through the air. 



We have seen that a species may under new condi- 

 tions of life change its habits, or have diversified habits, 

 with some habits very unlike those of its nearest con- 

 geners. Hence we can understand, 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 

 more than enough to stagger any one ; yet in the case 



