INTRODUCTION. 



13 



in many cases. I can offer no better example of this dis- 

 regard for phylogenetic and convergent evolution than is 

 shown in the proposed scheme of classification of the 

 Cecidomyidse on later pages. The structure and use of 

 the organs of orientation, — the antennae and palpi especial- 

 ly, the further comparative study of the ocelli and eyes, 

 the reasons for the evolution of the wing venation in ap- 

 parently different systems, the causes of the variations in 

 the patterns of coloration, the meaning more fully of the 

 different kinds of vestiture, etc., etc., all need much more 

 attention than has been given them by the systematise 

 and it is he who is best qualified to solve such problems. 

 One of the first questions that a novice in classification 

 asks is: What is a species, genus, family? The taxono- 

 mist's answer to the first of these queries is easy: A s f pe- 

 cies is a form of life with all its fertile variants. A mas- 

 tiff and a grayhound are not distinct species of clogs, 

 because there exist all possible variations between the 

 two types, though both have bred true to themselves for 

 more than three thousand years. But a dog and a fox 

 are distinct species because there exist no varieties con- 

 necting the two. If no two specimens in a given form of 

 fly have precisely the same relative lengths of the anten- 

 nal joints, then the relative lengths of these organs is not 

 a specific character in that form. If, however, all the 

 specimens occurring in Massachusetts have a definite 

 relative length for each joint while those in Kansas have 

 another, the first impression is that they belong to dis- 

 tinct species. If further discovery proves that, in cross- 

 ing the country between Massachusetts and Kansas, the 

 lengths gradually vary from one to the other, then we 

 must consider the eastern and western specimens as mere- 

 ly racial varieties of a single species. The systematist is 

 never troubled as to what a species is, if he has all the 

 material he wants. 



