50 Resemblances and Differences Among Living Things 



and beetles and bugs and mosquitoes. One may know several 

 kinds of butterflies apart, several kinds of cats, several kinds 

 of pine. The more our knowledge grows, the more discrimi- 

 nations do we make. We come to need more names for re- 

 cording our knowledge, for thinking about the many kinds 

 of objects we know, and for the communication of our ideas 

 to others. 



It is the mark of ignorance to insist that " pigs is pigs," 

 that bugs are bugs, or that trees are trees. At the same time 

 most of us grow up in the habit of thinking about a species 

 as something fixed, something definitely limited, something 

 sharply separated from other species. 



At the level of knowledge which is satisfied to call all 

 creepy things bugs or spiders it is not necessary to define 

 precisely what we mean by a ki^td or a species. But when 

 we know enough to start making precise definitions we find 

 that our knowledge stands in the way. That is, the more 

 discriminating one is, the more difficult is it for him to tell 

 exactly what to name a given plant or animal. Whereas 

 the purpose of a species name is to bring together all the in- 

 dividuals that are exactly alike, we are dismayed to discover 

 that the name not only includes individuals that are markedly 

 different from one another, but is often inadequate since a 

 given individual " belongs " just as clearly to two or even 

 more species! 



What is a Worm? 



The essential problems of classification may be illustrated 

 from a common experience. For many people a ** silk- 

 worm " is merely a " kind of worm." Of course everybody 

 knows what a worm is. Yet the more one finds out about 

 ^' worms " the less satisfactory is the name, for it includes 

 altogether too many radically different kinds of animals. It 

 is much more than splitting hairs to say that the " silk- 

 worm " is not a worm at all, but a caterpillar. The dis- 

 tinction is very real, and may have important practical 



