THE EVOLUTION OF INSTINCT 133 



spin a cocoon, there are frequently individuals which fail to 

 spin, and apparently they suffer no injury from this lack of 

 covering. It is said, by many writers, that there is great 

 variation in the pugnacity of ants, some individuals attacking 

 all sorts of enemies with the greatest ferocity, while others 

 are so cowardly that they flee on the least intimation of 

 danger. In honey bees some forms have the instinct of 

 making drone cells greatly exaggerated and there are great 

 variations in the pugnacity of different stocks. 



Careful studies which have been made of the behavior of 

 the crayfish, the earthworm, Hydra and various protozoa 

 have revealed a surprising degree of individual variability. 

 In the field of tropisms striking variations are frequent 

 among animals subjected to the same external conditions. 

 Passing to higher organisms we find numerous records of 

 the variation of instinct, especially in birds and mammals. 

 Variations in the nest building of birds are common, and 

 numerous instances have been compiled by Darwin in his 

 posthumous essay on instinct. Bechstein states that in the 

 nightingale some individuals show an inherited tendency 

 to sing in the daytime instead of at night. There are many 

 instances in which birds have apparently lost the instinct 

 of migration. Certain breeds of domestic fowl have lost the 

 instinct to incubate their eggs and among other breeds this 

 instinct is notoriously variable. 



Trainers of animals frequently remark upon the striking 

 differences which are presented not only in aptitude for learn- 

 ing but in the habits and disposition of different individuals. 

 Yerkes finds in the dancing mouse marked differences in 

 sensitiveness to visual, auditory, tactual and olfactory 

 stimuli in individuals of the same age and sex; striking differ- 

 ences also occur in their general habits and in docility. 

 The peculiar dancing proclivity of this variety is of uncertain 



