THE GROWTH OF GROUPS u 



group which has come down unchanged from the Tertiary 

 epoch. It is important to know whether this opinion 

 is correct. It must be admitted that there is no reason 

 for the assumption that every species contains an infinite 

 and therefore negligible number of individuals. The 

 museum worker who defines most of the species has little 

 opportunity of forming an opinion as to the number of 

 individuals which are represented by a given type speci- 

 men. The collector who supplies material to the museum 

 is often able to form an opinion, but usually his attitude 

 is humble. If the edict has gone forth that such and such 

 a group is a species then henceforth it is and the matter 

 is at an end, at least for the time. But when an inquirer 

 fixes his attention on a group such as is called generic, 

 and examines thousands of individuals belonging to 

 that group extended over a wide area, he will come to 

 know that groups called specific are very variable in the 

 number of their component individuals, that some of them 

 contain only a few thousands which are confined to a 

 limited area. Few groups have been examined in this 

 manner. The most complete inquiry of the kind, in 

 the writer's knowledge, is Mr. W. L. Tower's investigation 

 of the potato beetle, Leptinotarsa, in America, which 

 extended over ten years. Most important are the 

 results of the breeding experiments which were under- 

 taken, but these will be referred to later on. The ques- 

 tion to be considered here is how were the beetles found 

 in nature ? A rare form called pallida was found, gene- 

 rally as single specimens isolated among great numbers 

 of a commoner species, but on one occasion six specimens 

 of pallida were found together ; six is a group, but it is 

 not what we call a species. Another form called rubicunda 



