BIOMETRIC IDEAS AND METHODS 61 



ter, and based on the observation of relatively 

 few individual cases, the significance of probable 

 error determinations in that field is clear. Simi- 

 larly, in the experimental study of inheritance 

 along Mendelian lines the results, as has been 

 pointed out in the preceding essay, are quantita- 

 tive and statistical in character. A statement of 

 such results without probable errors is incomplete. 

 In the third place biometry gives us a method of 

 measuring the relationship between the phenom- 

 ena, in the multitude of cases where this rela- 

 tionship is not of a simple cause and effect sort. In 

 observed biological phenomena there very often 

 exists a correlation rather than a strictly causal 

 relation between events or characters. The reason 

 for this doubtless lies in the' fact that in biology 

 we are dealing in most instances with complex 

 phenomena. As the complex phenomena are an- 

 alyzed into their simple (or, perhaps better, simpler) 

 components, correlation passes over into causation. 

 It is, however, a great aid in making such analyses 

 to have a method of measuring exactly the degree 

 of correlation which exists between observed 

 phenomena or characters. Of such methods 

 current biometrical technique furnishes a great 

 wealth. It is possible now to determine the de- 

 gree of correlation or association existing not only 

 between physically measurable characters, but 

 also between qualitative characters not capable of 

 precise measurement. 



