STRUCTURE AND DEVELOPMENT 39 



tory of steam vessels. How the members of the seal 

 tribe have changed in their descent from purely terres- 

 trial ancestors is partly explained by such intermediate 

 animals as the otter. This form is adapted by its 

 slender body and partly webbed feet to a semi-aquatic 

 life ; it seems to have halted at a point beyond which all 

 of the seals have passed in their evolution. 



Each one of these tribes by itself provides conclusive 

 evidence of evolution, for it is most reasonable to regard 

 the "theme" in every case as a product of common 

 inheritance, while the variations of any theme are best 

 understood as the results of adaptive changes in various 

 directions. But the examples have disclosed a larger 

 relation and a principle of wider scope, as indeed the 

 assignment of all these tribes to the single natural 

 group of the carnivora implies. These tribes are put 

 together because comparative anatomy finds that the 

 common characters of all cats are fundamentally like 

 those of all dogs and bears and seals, and in these com- 

 mon qualities the carnivora differ from all other mam- 

 malia. Does this mean that the branches which bear 

 respectively the various members of the several tribes 

 are outgrowths of a single limb of the evolving animal 

 tree ? Science does not hesitate to give an affirmative 

 answer, because, as in the case of the similar but vary- 

 ing domestic cats, no other explanation of tribal re- 

 semblance in structure seems so reasonable and natural. 



So far the examples have been taken from one order 

 of the highest class of backboned animals, called mam- 

 malia. When our survey is extended to other divisions 

 of this class, additional laws of organic relationship are 

 discovered. If in a series of evolving generations the 



