Variation in Hooks of Dog-Tapeworms 25 



is the smallest in every case, and it is hence the least variable of 

 all the parts. Next in order comes c in three out of the four 

 cases, while d appears to be the most variable of the characters. 



With exception of the figures given by Kiichenmeister (1852), 

 the mean magnitude of the large hooks of T. scrrata, as observed 

 at Lincoln, is above the upper limits given by the authors quoted 

 on page 12, Part I. It is different in the case of the small hooks, 

 however, the mean magnitude here being not far above the aver- 

 age of the mean found by above-quoted authors. 



In the case of T. scrialis, the minimum of the large hooks is 

 practically the minimum observed elsewhere, while the maximum 

 is far above the maxima given on page 12, Part I. The small 

 hooks average somewhat less. 



FRATERNAL, CORRELATION 



A question which is of great importance now arises : How 

 large a proportion of the total variability does each head exhibit? 

 If, by the standard deviation of an individual, we mean the aver- 

 age of the standard deviations of the individual heads, as found 

 from tables I and II, we have the following results : 



Taenia scrrata. — The standard deviation of the individual large 

 hook is 4.9, while the standard deviation of all the hooks is 9.62, 

 whence the individual variability is about 51 per cent of the 

 whole. 



The standard deviation of the individual small hook is 4.1, 

 while that of the type is 7.59, or the individual here exhibits 54 

 per cent of the total variability. 



Taenia scrialis. — For the large hooks we find the individual 

 variability to be 4.1, and the type variability to be 7.07, or 58 

 per cent of the total variability is found in the individual. 



The small hooks give us a mean standard deviation of 4.6, 

 which is 65 per cent of the total variability. If, however, we 

 omit the evidently abnormal case (6), we have the mean S. D.' 

 =3.6. and the individual variability is 51 per cent of the total 

 variability. 



215 



