16 DIFFICULTIES 



infinitely more perfect; for possibility of selection 

 a perfect (?) gradation is required. Different groups 

 of structure, slight gradation in each group, every 

 analogy renders it probable that intermediate forms 

 have existed. Be it remembered what strange meta- 

 morphoses ; part of eye, not directly connected with 

 vision, might come to be [thus used] gradually worked 

 in for this end, swimming bladder by gradation of 

 structure is admitted to belong to the ear system, 

 rattlesnake. [Woodpecker best adapted to climb.] 

 In some cases gradation not possible, as vertebrae, 

 actually vary in domestic animals, less difficult 

 if growth followed. Looking to whole animals, a bat 

 formed not for flight 1 . Suppose we had flying fish 2 

 and not one of our now called flying fish preserved, 

 who would have guessed intermediate habits. 

 Woodpeckers and tree-frogs both live in countries 

 where no trees 3 . 



The gradations by which each individual organ 

 has arrived at its present state, and each individual 

 animal with its aggregate of organs has arrived, 

 probably never could be known, and all present 

 great difficulties. I merely wish to show that the 

 proposition is not so monstrous as it at first appears, 

 and that if good reason can be advanced for believing 

 the species have descended from common parents, 

 the difficulty of imagining intermediate forms of 

 structure not sufficient to make one at once reject 

 the theory. 



1 This may be interpreted " The general structure of a bat is the same 

 as that of non-flying mammals." 



2 That is truly winged fish. 



3 The terrestrial woodpecker of S. America formed the subject of a paper 

 by Darwin, Proc. Zool. Soc., 1870. See Life and Letters, vol. iii. p. 153. 



