Raymond Pearl and Wilbur N. Fuller 
219 
(c) The average length of the individual somite in this sample of worms is 
evidently 
191-71 , ^, 
142715 = 1"^*"^™- 
This value so obtained accords very well with observation on the single somites. 
Its significance from a taxonomic standpoint will be considered later. 
We may turn next to the question of the correlation between total number of 
somites and length of body. The correlation surface is exhibited in Table III. 
Computing the value of the coefficient of correlation from this table by the 
usual equation 
^, ^ S {xy ) 
we get r = -260 + -028. 
This is a rather low coefficient and indicates that, so far as can be judged from 
this sample, there is no very close relationship between length and number of 
somites in the earthworm. 
It is seen at once by mere inspection that the relationship between the two 
characters is not the same in all parts of the correlation table. Thus the worms 
in the two lowest length classes (i.e. from 10 to 15 cm. in length) are not, in 
general, individuals having few somites, but instead these arrays centre well 
towards the right end of the table. The five worms having the smallest number 
of somites all fall in or above the third length class. 
This apparently paradoxical result of low correlation between length and 
number of somites is evidently to be explained as a consequence of the fact that 
the length of a worm depends not only on the number of somites in the body, 
but also on the size of the individual somites. It is the same sort of result as 
would be expected if the sitting height and number of vertebrae in man were 
correlated. 
This factor of increase in length of the individual somite can now be analysed 
somewhat more precisely, through the medium of the regression equations. For 
the regression of length on number of somites we have the coefficient of regression: 
h = -260 X = -0675, 
and for the regression of number of somites on length the regression coefficient 
6, = -260 x^^=1001. 
The characteristic equations, referred to the means as oi'igin, are 
(i) L = 9-538 + -0675 S, and 
(ii) >S = 123-525 + 1-001 X, 
