12 THE GROWTH OF GROUPS 



composed a group which was much larger than this, but 

 still it was limited. Thus, in August, 1903, it was found 

 over an area of some seven or eight square miles, but in 

 September, 1906, it was limited to a tract of about ten 

 acres in extent, where only a very few specimens were 

 found, hence the group was waning. The better known 

 form decemlineata was first recorded in Nebraska in 1859, 

 in all probability it was then occupying a limited area, 

 at all events it was limited in comparison with the im- 

 mense area which it came to occupy in the following fifty 

 years. Taking the potato plant as its chief food it spread 

 from Nebraska eastward as far as the Atlantic border, 

 travelling at about the rate of sixty miles a year. The 

 potato beetle is of great economic interest, consequently 

 its distribution was carefully recorded. The relative 

 importance of pallida, rubicunda, and decemlineata are 

 known, but in the case of other generic groups which are 

 not so well known the component species may have the 

 same relative importance as those mentioned and yet be 

 considered as of equal value, since each is represented by 

 a type specimen and a verbal description. The only 

 difference in our regard being that a group such as pallida 

 would be considered rare, while a group such as decem- 

 lineata would be common. Now, it is well known that 

 " rare " species are very plentiful. The verbal paradox, 

 why are rare species so common, has occurred to many 

 naturalists both of the past and present. It seems that 

 pallida, rubicunda, and decemlineata are all typical of 

 species so-called, and if we wish to explain how animals 

 came to be as they are we must not shut our eyes to such 

 groups as pallida and rubicunda, on the grounds that they 

 are not exactly species since we know too much about them. 



