WHAT WE OWE TO DARWIN 23 



forms. When we take a particular group of 

 animals it is often possible to draw that branch 

 of the genealogical tree with a firmer hand, for 

 it is the mutual relations of the large series which 

 are so difficult. From the actual classification of 

 organisms, from the peculiar way in which the 

 categories are related one within the other 

 species, genera, families, orders, classes we get 

 an impression of affiliation which we do not get 

 from a classification of rocks or other inanimate 

 objects. 



(c) Darwin attached great importance to the 

 anatomical evidence of adherence to general type 

 in spite of the most manifold diversity in external 

 form. If we take, for instance, a series of fore- 

 limbs among Vertebrates the arm of a frog, the 

 paddle of a turtle, the wing of a bird, the fore-leg 

 of a horse, the flipper of a whale, the wing of a 

 bat, the arm of man we find a detailed homology 

 not only as regards the bones, but as regards 

 muscles, nerves, and blood-vessels. It is difficult 

 to suggest any interpretation except that the 

 resemblance is due to relationship. As Darwin 

 said : " How inexplicable is the similar pattern 

 of the hand of a man, the foot of a dog, the wing 

 of a bat, the flipper of a seal, on the doctrine of 

 independent acts of creation ! How simply ex- 

 plained on the principle of the natural selection 

 of successive slight variations in the diverging 

 descendants from a single progenitor ! " 



(d) Darwin made a good case out of rudi- 

 mentary or vestigial organs the dwindling remains 

 of structures which were presumably well developed 

 in ancestral forms. Cetaceans have no visible 

 hind-limbs, but many show relatively small ves- 



