HISTORY AND NATURAL HISTORY 47 



lack sexual significance. There is some protection in 

 the fact that if one is disturbed the whole group may 

 be warned. The presence of many butterflies would 

 reinforce any species odor that might attract others 

 of the same species, or repel possible predators. 



The crowded roosts to which certain birds return 

 not only for one season but sometimes for years are 

 widely known. Here again we are concerned with a 

 positive social appetite which grows stronger with 

 the approach of darkness; the details as to why and 

 how it operates are not known. 



Animals which come together in intermittent 

 groupings like these overnight aggregations are 

 showing a social appetite which is none the less 

 real because it is effective only at spaced intervals. 

 In this it resembles other appetites such as those 

 for food, water and sex relations. From such occa- 

 sional or cyclic expressions of a social appetite it is 

 a relatively short step to whole modes of life which 

 are dominated by a drive for social relationships. 

 As I have already said, in the insects alone this step 

 has been taken some twenty-four distinct times and 

 in widely separated divisions of that immense group. 



Normally the development of highly social life 

 comes by way of an extension of sexual and family 

 relations over greater portions of the life span. 

 Here again all degrees of increased length of asso- 



