INTRODUCTORY 13 



has no essential dependence on the nature of the characteristic 

 which manifests them. It is for example sometimes said that 

 colour-distinctions are of small systematic importance, but every 

 systematist is familiar with examples (like that of the wild species 

 of Callus} in which colours though complex, show very little 

 variation. On the other hand features of structure, sexual dif- 

 ferentiation, and other attributes which by our standards are 

 estimated as essential, may be declared to show much variation 

 or little, not according to any principle which can be detected, 

 but simply as the attention happens to be applied to one species 

 or group of species, or to another. In many groups of animals and 

 plants observers have hit upon characters which were for a time 

 thought to be finally diagnostic of species. The Lepidoptera and 

 Diptera for instance, have been re-classified according to their 

 neuration. Through a considerable range of forms determinations 

 may be easily made on these characters, but as is now well 

 known, neuration is no more immune from variation than any 

 other feature of organisation, and in some species great varia- 

 bility is the rule. Again it was once believed by some that the 

 genitalia of the Lepidoptera provided a basis of final determination 

 with a similar sequel. In some groups, for example the Lycae- 

 nidae, or the Hesperidae, there are forms almost or quite in- 

 distinguishable on external examination, but a glance at the 

 genitalia suffices to distinguish numerous species, while on the 

 contrary among Pieridae a great range of species show scarcely 

 any difference in these respects: and again in occasional species 

 the genitalia show very considerable variations. 



The proposition that animals and plants are on the whole 

 divisible into definite and recognisable species is an approxi- 

 mation to the truth. Such a statement is readily defensible, 

 whereas to assert the contrary would be palpably absurd. For 

 example, a very competent authority lately wrote: "In the 

 whole Lepidopterous fauna of England there is no species of 

 really uncertain limits." 7 Others may be disposed to make 

 certain reservations, but such exceptions would be so few as 

 scarcely to impair the validity of the general statement. The 



7 J. W. Tutt, in Ent. Rec., 1909, XXI, p. 185. 



