278 



EVOLUTION 



different, the former resembling that 

 of South America while the fauna of 

 the latter resembles that of Africa? 



The second set of observations re- 

 lated to the fact that as he traveled over 

 South America he noticed that the 

 species occupying a particular niche 

 in some regions were replaced in neigh- 

 boring regions bv other species that 

 were different, }et closely similar. 



The third set of observations was 

 concerned with the fact that in the 

 pampas he found fossil remains of 

 large mammals covered with armor 

 like that of the armadillos now living 

 on that continent. Whv were these 

 extinct animals built on the same plan 

 as those now living? 



On the view that species were im- 

 mutable and had not changed since 

 they were severally created, there was 

 no rational answer to any of these ques- 

 tions. On the other hand, if species, 

 like varieties, were subject to modifica- 

 tion during descent and to divergence 

 into different lines of descent, all these 

 questions could be satisfactorily and 

 simply answered. 



In possession of a working hypoth- 

 esis that species have undergone evo- 

 lution and succesive origination by de- 

 scent, with modification, from ancestral 

 species shared in common with other 

 species, Danvin next proceeded to 

 search the whole field of botanical and 

 zoological knowledge for evidence 

 bearing on his hypothesis. He realized 

 that no general principle that explained 

 the evolution of animals was accept- 

 able unless it also applied to plants. 

 The result was one of the most re- 

 markable attacks on a problem ever 

 made by the inductive method of 

 searching for facts, whatever their im- 

 port might be. 



In the first place, in cultivated 

 plants and domestic animals such as 

 the dahlia, the potato, the pigeon, and 

 the rabbit, a large number of varieties 

 have in each case been produced from 



a single original stock. Descent with 

 modification and divergence into sev- 

 eral lines is therefore certainly possible 

 within the species. 



Comparative anatomy reveals the 

 existence of similar plans of structure 

 in large groups of organisms. Plants 

 may have vegetative leaves, and in some 

 cases these are modified into parts of 

 flowers. Vertebrate animals have fore- 

 limbs that may be used for walking, 

 running, swimming, or flying, but in 

 which the various parts of the skeleton 

 correspond, bone for bone, from the 

 upper arm to the last joints of the fin- 

 gers, whether the animal is a frog, a 

 lizard, a turtle, a bird, a rabbit, a seal, 

 a bat, or a man. Tliis is what is meant 

 by saying that such structures are 

 homologous, and these correspond- 

 ences are inexplicable unless the ani- 

 mals are descended from a common 

 ancestor. Fundamental resemblance is 

 therefore evidence of genetic affinity. 



Embryology reveals remarkable 

 similarity in structure between )oung 

 embr}'OS of animals which in the adult 

 stage are as different as fish, lizard, 

 fowl, and man. This similarity even 

 extends to such details as the manner 

 in which the blood vessels run from the 

 heart to the dorsal aorta, a plan which 

 is of obvious significance in the case of 

 the fish that breathes by means of gills, 

 but not so obvious in that of lizard, 

 chick, or man, where gill pouches are 

 formed in the embryo but soon be- 

 come transformed into different struc- 

 tures, and breathing is carried out bv 

 other means. This similaritv between 

 embryos is explained by the affinity and 

 descent from a common ancestor of 

 the groups to which thev belong. 



Embr}olog\' also provides evidence 

 of vestiges of structures which once 

 performed important functions in the 

 ancestors but now either perform dif- 

 ferent functions or none at all. Ex- 

 amples of such organs are the teeth 

 of whalebone whales, the limbs of 



