Vol. 1.] Wooclwortli. — Wing Veins of Insects. 87 



types directly in the groups toward which they are develop- 

 ing, and deny to them the status of transitional forms not yet 

 sufficiently differentiated to be separated one from the other. 



No one will seriously deny the contention that any really 

 natural s_ystem of classification must be one that is natural at 

 every stage of phylogenetic development from the earliest time 

 to the present, and that it is proper to shut one's eyes to the 

 later developments when treating the earliest forms. With 

 equal truth it can be maintained that groups represent the 

 product of two factors working in their evolution, of which one 

 consists of those forces or conditions accountable for divergence 

 in structure, and the other of those natural conditions making 

 intermediate forms untenable. The greater groups are those 

 separated by wide, impassable gulfs, and the lesser b}' nar- 

 rower but none the less impassable gulfs. As soon as species 

 are distinct, that is, as soon as interbreeding is impossible, 

 they become as truly isolated as though the differences were 

 of a more profound character. Classification therefore repre- 

 sents not so- much the construction of a genealogical tree as 

 the expression of the natural barriers which set the bounds to 

 morphological differentiation. 



According to the present practice of nomenclature we have 

 to recognize in the primitive winged insect, for instance, a 

 species, genus, family and order. Its first differentiation that 

 became distinct would be a new species within the same 

 genus, family and order. A new species would he, considered 

 as established as soon as a form became infertile or incapable 

 of breeding with other forms of the original species. The 

 total amount of divergence from the original form has no nec- 

 essary relation to the question, the whole matter being depend- 

 ent upon those items of differentiation which influence or affect 

 the fertility or capacity for interbreeding. 



For this reason it is evident that the whole species may 

 develop away from the original type to any extent and remain 

 one species, though not necessarily the same species as -the 

 primitive one. It may also vary to an extreme degree, but as 

 long as the fertility between the individuals remains there is 

 but a single species. Such extreme variation may either produce 

 an inconstant polymorphism, or may be associated with chang- 

 ing external conditions, the insect becoming sensitive to them, 

 producing phytophagic forms, seasonal variation, or alterna- 



