52 Resemblances and Differences Among Living Things 



When we see a particular individual living thing in the 

 field, we are of course unable to tell very much about its 

 ancestors. For all we know, that worm which we see crawl- 

 ing around the edge of that stone never had any parents or 

 any other ancestors. For all we know, it may have been 

 created right there just before we appeared upon the scene. 

 We may have positive knowledge about the origin of certain 

 individual plants and animals. That dog is one of a known 

 litter; we grew that corn from seed saved from last year, and 

 so on. From this limited knowledge we extend our interpre- 

 tation to all living things. We manage our affairs as if every 

 living thing did actually come from parents, and as if all of the 

 same *' kind " did actually have common ancestors (Fig. 1 1 ) . 



How are Species Related? 



How is it, however, when we compare plants and ani- 

 mals that do not resemble each other so closely? For ex- 

 ample, the fox and the dog are different enough to get 

 different names from ordinary observers. Why do we never- 

 theless believe that they are ** related "? In the case of human 

 families, we speak of relationship only, as a rule, when we 

 know something of the actual facts as to parents, grand- 

 parents, and so on. We should not ordinarily be satisfied to 

 assert a relationship between a Mr. Morton in one state and 

 a Mr. Lee in another state merely because both happen to 

 have hair of the same color, or because we notice a resem- 

 blance in the shape of the mouth. 



What warrant is there for saying that any species of 

 animal is related to another? It is here of course impossible 

 to obtain family histories. We have to find our evidence in 

 other directions. We use in the first instance the same as- 

 sumptions that we make when we call two distinct plants 

 by the same name, for example, two pine trees, two violets; 

 or when we say of a particular animal the first time we 

 see it, " that is a sheep," or " that is a bear." The only 

 facts we know directly are particular individual objects — 



