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 
