148 ANIMAL LIFE AND SOCIAL GROWTH 



ble steps of increasing parental care can be traced 

 from even more humble beginnings through differ- 

 ent stages of increasing length and degree of 

 protectiveness to their culmination in the extreme 

 human cases where a doting father may provide 

 trust funds for his children in attempted 

 perpetuity. 



With this criterion, or with any other suggested 

 for limiting social life, there are difficulties in 

 application. The young nestling birds and their 

 parents, according to this definition, are mem- 

 bers of a social group as long as they continue 

 to live together; but if, as in the case of the cow- 

 birds, the young are hatched and reared singly 

 as social parasites in the nests of other species, 

 then the compact flocks which such birds form 

 by the collection of these nestlings which were 

 reared in isolation from others of their species, 

 would not be considered a social group although 

 otherwise it presents all the characteristics of a 

 closely integrated bird flock. The difficulties 

 of precise definition are great for one who tries 

 to delimit the more closely knit societies from 

 their loosely organized forerunners. 



One of the most easily applied tests of the 

 degree of integration of a social group is the extent 

 to which the animals co-operate with one another 

 to accomplish a common end. There is good 



