104 The American Naturalist. [February, 



The anticipation naturally intrudes itself that the charac- 

 ters which distinguish the steps in a single evolutionary or 

 genealogical line must disappear with discovery, and new ones 

 appearand that they must be all variable at certain geological 

 periods, and hence must become valueless as taxonomic criteria. 

 And it is therefore concluded that our systematic edifice must 

 lose precision and becomes a shadow rather than a reality. I 

 think that as a matter of fact this will not be the result, and for 

 the following reasons. In the first place, when, say all the gen- 

 eric forms of a genealogical line, shall have been discovered, 

 we will find that each one of them will differ from its neigh- 

 bor in one character only. This naturally follows from the 

 fact that two characters rarely, if ever, appear and disappear 

 contemporaneously. Hence, generic characters will not be 

 drawn up so as to include several points. For a while, there 

 will be found to be combinations of two or three characters 

 which will serve as definitions, but discovery will relegate 

 them to a genus each. Each of these characters will be found 

 to have what I have called the " expression point," or the 

 moment of completeness, before which it cannot be said to ex- 

 ist. In illustration I cite the case of the eruption of a tooth. 

 Before it passes the line of the alveolus it is not in use ; it is 

 not in place as an adult organism. When it passes that line 

 it has become mature, has reached its expression point, comes 

 into functional use, and may be counted as a character. Such 

 will be found to be the case with all separate parts ; there al- 

 ways will be a time when they are not completed, and then 

 there will be a time when they are. These lines, then, will 

 always remain as our boundaries, as they are now, for all nat- 

 ural divisions from the generic upwards. This condition can- 

 not exist in characters of proportionate dimensions, which 

 which will necessarily exhibit complete transitions in evolu- 

 tion. Hence, proportions alone can only be used ultimately 

 as specific characters. 



Some systematists desire to regard phyletic series as the only 

 natural divisions. This may be the ultimate outcome of pale- 

 ontologic discovery, but at present such a practice seems to me 

 to be premature. In the first place, as all natural divisions 



