54 Resemblances and Differences Among Living Things 



particular plants, or animals, or persons. However many ac- 

 quaintances you may have, you never know " mankind " or 

 " the human race." We know only single persons. We group 

 individuals into species on the basis of their resemblance. 

 We can group species together — and assume " relationship " 

 in the sense of common ancestry — on exactly the same 

 basis. 



The other side of this problem is seen when somebody 

 points out consistent differences among individuals that we 

 have been lumping together under one name. Consider, for 

 example, the very numerous plants, growing over a large 

 area, that you and I call " violets " and that we assume to 

 be " related." Along comes a botanist and tells us that all 

 of these plants which we have been satisfied to call by one 

 name represent at least twenty different " species." When 

 he calls our attention to the details we acknowledge that the 

 differences are clear enough. Are all of the so-called ** vio- 

 lets " still to be considered *' related " even if we agree to give 

 special or specific names to each of the twenty or more differ- 

 ent ** kinds " that we can distinguish? 



Ordinary thinking includes the suppositions that ( i ) all 

 the individuals that resemble one another sufficiently are 

 members of the same species; and that (2) all the members of 

 a species have a common ancestry. The idea of organic 

 evolution includes the further supposition that several such 

 groups-of-individuals or " species " may also have a common 

 ancestry. 



If we cannot trace back the family histories, we can at 

 least consider the question of how close a resemblance there 

 is between the red maples and the sugar maples for example. 

 We can then try to decide whether the resemblances are 

 sufficient to satisfy the assumption that all of them have de- 

 scended from a common ancestor. If we compare an eagle 

 with a jellyfish we find the differences so striking that we have 

 difficulty in finding similarities at all. If we go further 

 and try to find out what all living things have in com- 

 mon, numerous resemblances — and corresponding sugges- 



