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- 



