27^ THE SOCIAL LIFE OF ANIMALS 



have traveled to reach their present state of highly 

 developed social life. 



We cannot outline the steps taken very closely, but 

 it would seem that in this cockroach-termite stock 

 aggregations allowed aspects of mass physiology to 

 develop which in turn permitted a closely knit and 

 varied social evolution. This is about as near as we 

 have yet been able to come to charting a direct and 

 obvious truly social development from a slightly so- 

 cial or sub-social animal aggregation. 



Among grasshoppers crowding can produce obvi- 

 ous structural changes (Figure 49). Certain species of 

 grasshoppers found in semi-arid regions, such as 

 those of South Africa, have two phases (5) that are 

 quite distinct from each other. The phases are suffi- 

 ciently different so that in the past they have been 

 described as being different species. There is at 

 present much evidence which indicates that the 

 phase solitaria can be turned into phase gregaria by 

 crowding the young nymphs into dense masses. The 

 opposite transformation may take place when the 

 nymphs of phase gregaria are reared under un- 

 crowded conditions. The differences between the two 

 extend into color, form and size. 



Similarly plant-lice, which are also called aphids, 

 exist in wdnged and wingless forms which tend to 

 alternate. When the wingless aphids have approxi- 



