328 
On the Nest and Eggs of the Common Tern 
colour, whether green or brown, with either size or shape of egg*. This does not 
appear at all unreasonable if we assume the ground colour to be deposited before 
the egg enters the oviduct or the shell becomes finally hardened. 
The general conclusion therefore to be drawn from the present investigation is 
that intensity of ground colour, whether green or brown, has no relation to egg 
size and shape, but that breadth of egg, Avhether considered directly or through 
the index, is more probably related though not intensely to mottling, but the 
nature of the relationship must be obscure until a more elaborated classification of 
mottling has been adopted. 
(iii) Relation of Mottling to Ground Colour. 
The data are given in Table G"* at the end of this memoir, where we have 
separated brown from green eggs, because it is conceivable that the relationships, 
if any, for the two categories might be different. If G.^ denote the mean contin- 
gency when there is zero association we have : 
For the Brown Eggs : = -2830 + -0323. 
Uncorrected Contingency : C'„ = '2030. 
For the Green Eggs : a = -2118 ± -0261. 
Uncorrected Contingency : G., = -2557. 
Thus for the brown eggs there is no significance in C^, it being less than the 
mean value of the contingency, when there is no association. For the green eggs 
Co is greater than C., but the difference is less than twice the probable error of Oj; 
we cannot therefore assert any real relation to exist between mottling and intensity 
of ground colourf. Under the circumstances of the above relation of G.^ to G2, it 
did not seem necessary to correct C., as such correction would not alter the con- 
clusion of no significant association. Although the intensity of ground colour 
may have no relation to mottling, it is conceivable that the colour of the egg may 
itself have relation to mottling or indeed to intensity of ground colour, i.e. a brown 
egg may have deeper tones of ground colour and denser mottling than a green egg. 
We have the following biserial tables to illustrate these cases. 
TABLE XV. 
Mottling and Colour of Egg. 
Mottling Categories. 
Colour of Egg 
g + d 
a + h 
c + h 
/ 
Totals 
Brown 
89 
29 
215 
57 
11 
437 
Green 
61 
148 
39 
300 
98 
23 
669 
Totals ... 
97 
237 
68 
515 
155 
34 
1106 
* This statement is not really contradicted by tbe 7;=-150(j of p. 148 of the previous memoir, for 
with the small number 291 eggs of that census 7?o= -1655 ± -0395 !, so that is less than the value for 
zero association. 
t Examined in the same manner the result for 1913 appears not to have the significance we 
attributed to it. We have 0^= -asiS ± -0395, while the corrected contingency is only C2 = -2260. Thus 
C-> is actually less than the mean value when there is no contingency. 
