38 GENETICS 



unique, because nature herself, whom he strives to 

 follow, never duplicates anything. 



The Bertillon system of personal identification is 

 based upon the constancy of minor variations found 

 in each individual. Its importance is shown in 

 Figure 24. The faces of the criminals there pictured 

 would be easily confused by the ordinary observer, but 

 an examination of their thumb prints shows unmis- 

 takable differences between these three individuals. 



3. Kinds of Variation 



A brief enumeration of some of the kinds of varia- 

 tion will reveal their diverse character. 



a. With respect to their nature variations may be 

 morphological, physiological, or psychological. Under 

 morphological variations are included differences in 

 shape, size, or pattern as well as differences in number 

 and relation of constituent parts. 



Differences in activity are of a physiological nature. 

 Many animals in captivity are less fertile than when 

 free, while different individuals are well known to 

 vary widely with respect to their susceptibility to 

 disease. Nageli, for example, reports the presence of 

 tubercles in 97 per cent of the cases in 1^ve hundred 

 autopsies, although a majority of the deaths in ques- 

 tion was not due to tuberculosis at all, — a fact which 

 indicates a great diversity in the resistance of differ- 

 ent individuals to the tubercle bacillus. 



Psychological variations in man, such as those 

 which determine the disposition or mental traits of 

 individuals, are apparent to every one. 



