What is a Species? 51 



bearings. For one thing, a worm remains only a worm to 

 the end of its Hf e, no matter how long that may be, whereas 

 a caterpillar may in time become a moth or a butterfly. It 

 is, in fact, an early stage in the development of an en- 

 tirely different kind of animal. It is not necessary, however, 

 to sit by watchfully waiting in order to discover whether 

 the being in question is or is not a worm. Differences be- 

 tween a worm and a caterpillar are easily observable in the 

 outward appearance and in the character of the movements; 

 and the internal structures of the two forms differ still 

 more. 



By many people a small snake may be mistaken for a 

 " worm," and again we can understand the reason for such 

 a mistake. Yet anybody who is familiar with either worms 

 or snakes can tell you that the other animal does not belong 

 in the same class, notwithstanding the superficial resemblance. 

 It takes very little additional observation to reveal in the two 

 animals two distinct plans of structure. Indeed, the snake 

 differs more from the worm than does the caterpillar, and is 

 in all essentials more like an alligator, although you might 

 not guess this on first acquaintance. 



What is a Species? 



According to the traditional conception a species in- 

 cludes all the descendants of a common ancestor or of an 

 original pair of " the same kind." Without stopping to make 

 definitions, this is what most people do actually mean. This 

 supposed common ancestry is something that we can neither 

 know as a fact, nor in any sense " prove " ever to have 

 existed. It is, nevertheless, so reasonable an inference from 

 the facts which we do know about living things, that prac- 

 tically everybody takes it for granted. In the same way, 

 practically all people take for granted the common ancestry 

 of all human beings, whether we accept the story of Adam 

 and Eve or not, whether we accept any theory of evolution 

 or not. 



