THE GROWTH OF GROUPS 119 



on the potato beetles of America. In comparing my 

 results with his, I am fully conscious that as regards 

 completeness no comparison can be made. Even as a 

 survey my work is less complete than his, but the survey 

 of the beetles was merely preliminary to the far more 

 important breeding experiments which were carried out. 

 In the case of the rats no experiments, worth mentioning, 

 were made. However, the results of the survey of the 

 rats are in many ways like those of the more complete 

 survey of the beetles. In both cases sports and groups 

 of sports were found. The sports were of a limited 

 number of kinds, the same kind appearing in widely 

 separate places, some more often than others, for un- 

 known reasons. 



Mr. Tower's memoir was published in 1906 * and con- 

 tains the results of eleven years' work, it may well serve 

 as a guide to any one who wishes to inquire into the method 

 of evolution in a direct manner. A problem offered by 

 plants and animals is this how have they come to be 

 as they are ? The systematic zoologist who determines 

 how the animals are, their appearance at the moment, 

 does not as a rule inquire into this problem. On the other 

 hand, some of those who are particularly interested in 

 the origin of species, and endeavour to explain how 

 animals reached their present condition are not so well 

 acquainted with that actual condition. Mr. Tower, 

 however, commenced his work among the beetles with a 

 survey of the group, he ascertained the areas of dis- 

 tribution of the various species and sub-species. Some 

 of these areas were of great extent, comprising more 

 than the eastern half of the United States ; others were 



* Publications of the Carnegie Institute of Washington. 



