THE FOUNDATIONS OF GENETICS 267 



as much coffee as they desire without unpleasant effects. Walter 

 cites an interesting case in th(^ kea parrot of New Zealand. This 

 bird was herbivorous but became carnivorous after the introduc- 

 tion of sheep into its native home. Thus physiological variations 

 may occur in individuals as well as among the different individuals 

 making up a species. 



Psychological. Variations in mental qualities and processes 

 are evident in all of our associates. They too may be witnessed 

 in the individual from day to day as well as among the various 

 individuals of a group or species. 



To what extent they are separable from a morphological and 

 physiological basis is less evident than the association of those 

 types of variation; there are still many persons who look upon 

 mind as an independent phenomenon. Biologically they are the 

 variable expression of brain functions. That they are intimately 

 associated with other bodily processes is amply attested by the 

 effects of indigestion or any other slight illness upon mental 

 efficiency, which most human beings have unwillingly experienced. 

 In spite of our relatively meager knowledge of nervous functions 

 it is therefore logical to believe that our mental processes and 

 their variations are as definitely and completely associated with 

 them as are any other phenomena of vital activity with underlying 

 organic causes. 



Variations of Degree. According to degree two kinds of varia- 

 tions are commonly recognized: 



Continuous. Such variations grade through long series of 

 individuals without an apparent Ijreak or abrupt transition. The 

 dimensions of any individuals of a given species, for example, 

 usually vary gradually from minimum to maximum. The curli- 

 ness of human hair also varies by minute degrees from the straight 

 hair of the Mongolian races to the kinky hair of the negro. Curli- 

 ness is based on the form of hair ; straight hair is cylindrical while 

 kinky hair is distinctly flattened, and various intergrades occur. 



Discontinuous. Discontinuous variations, on the other hand, 

 exhibit abrupt transitions. Variations in the number of parts 

 of an organism are always of this type, for there can be no gradation 

 between six and seven petals or four and five rays. The term 

 has also been applied in an entirely different sense to mutations; 

 these are mentioned in the following pages under a different 

 category. 



