NATURAL SELECTION, ETC. 121 



geological considerations in favor of Darwin's hy- 

 pothesis has so extended as to leave no room for con- 

 sidering " the great facts of comparative anatomy and 

 zoology" with which Darwin's theory "very well 

 accords," nor for indicating how "it admirably serves 

 for explaining the unity of composition of all or- 

 ganisms, the existence of representative and rudimen- 

 tary organs, and the natural series which genera and 

 species compose." Suffice it to say that these are the 

 real strongholds of the new system on its theoretical 

 side ; that it goes far toward explaining both the 

 physiological and the structural gradations and rela- 

 tions between the two kingdoms, and the arrangement 

 of all their forms in groups subordinate to groups, all 

 within a few great types ; that it reads the riddle of 

 abortive organs and of morphological conformity, of 

 which no other theory has ever offered a scientific 

 explanation, and supplies a ground for harmonizing 

 the two fundamental ideas which naturalists and phi- 

 losophers conceive to have ruled the organic world, 

 though they could not reconcile them ; namely, Adap- 

 tation to Purpose and Conditions of Existence, and 

 Unity of Type. To reconcile these two undeniable 

 principles is the capital problem in the philosophy 

 of natural history ; and the hypothesis which consist- 

 ently does so thereby secures a great advantage. 



"We all know that the arm and hand of a monkey, 

 the foreleg and foot of a dog and of a horse, the wing 

 of a bat, and the fin of a porpoise, are fundamentally 

 identical ; that the long neck of the giraffe has the 

 same and no more bones than the short one of the ele- 

 phant ; that the eggs of Surinam frogs hatch into tad- 



