38 DOCTRINE OF EVOLUTION 



dences of relationship. The varieties of domesticated 

 dogs differ far more widely among themselves than do 

 common cats, yet their community of ancestry is 

 demonstrated not only by structural resemblances, but 

 also by the striking fact that forms as diverse as the 

 greyhound and the fox terrier can be crossed. Here 

 again there are wild forms, like the wolf and fox and 

 jackal, so like the domesticated members of the dog 

 tribe that we cannot fail to recognize a common '^dog- 

 ness" and its significance as evidence of the relationship 

 in ancestry of all these animals. 



Extending our survey so as to include the other 

 tribes of flesh-eaters, identical principles come to light. 

 One is compelled to regard the polar and grizzly bears 

 as obvious blood relatives of the brown bear, and even 

 of the raccoon of our own territory. Instead of walking 

 upon their toes like cats and dogs, these animals plant 

 their feet flat upon the ground ; and they agree in many 

 other details of structure that place them together, but 

 somewhat apart from the other tribes. The many 

 kinds of seals and walruses and sea elephants form 

 still another group displaying similar bodily characters, 

 but differing more widely from the '^cat theme'' in 

 these differences. They are all true carnivora, but in 

 the course of their evolution they have progressively 

 changed so as to be adapted to life in the water where 

 they find their prey. The bones of the limbs are the 

 same in number and arrangement as in the cat's limb, 

 but the seal's anterior appendage or ^^arm" has altered 

 in numerous ways so as to become an efficient flexible 

 paddle, while the hind limbs have shifted posteriorly, 

 very much as screw propellers have evolved in the his- 



