(.karacters as Adaptive and Specific. 253 



very common almost all over Europe, and ranges 

 from North America to the Himalayas, the number 

 of similar forms over this wide area would probably 

 have to be reckoned by hundreds, if not by thou- 

 sands 1 ." 



One or two further quotations may be given to 

 the same general effect, selected from the writings of 

 specialists in their several departments. 



" There is nothing that divides systematists more than what 

 constitutes a genus. Species that resemble each other more than 

 other species, is perhaps the best definition that can be given. 

 This is obviously an uncertain test, much depending on 

 individual judgement and experience ; but that, in the evolu- 

 tion of forms, such difficulties should arise in the limitation 

 of genera and species was inevitable. What is a generic 

 character in one may be only a specific character in another. 

 As an illustration of the uncertain importance of characters, 

 I may mention the weevil genus Centrinus, in which the 

 leading characters in the classification of the family to w^ich 

 it belongs are so mixed that systematists have been content 

 to keep the species together in a group that cannot be defined. 

 . . . No advantage or disadvantage is attached, apparently, 

 to any of the characters. There are about 200 species, all 

 American. 



The venation of the wings of insects is another example of 

 modifications without serving any special purpose. There is 

 no vein in certain Thripidae, and only a rudiment or a single 

 vein in Chalcididae. There are thousands of variations more 

 or less marked, some of the same type with comparatively 

 trivial variation, others presenting distinct types, even in the 

 same family, such genera, for example, as Polyneura, Tetli- 

 getra, Huechys^ &c. in the Cicadidae. 



Individual differences have often been regarded as distinctive 



of species ; varieties also are very deceptive, and races come 



very near to species. A South- American beetle, Arescus 



htstrio, has varieties of yellow, red, and black, or these colours 



1 Darwinism, p. 77. 



