150 On Homotifposh in Eggs of the Common Tern 
following are the orders (a) of mottliag chosen, (6) of breadth classes, (c) of 
index classes : 
(a) 
Assumed Order of 
Mottling 
Order of Breadth 
Order of Index 
Class 
B 
Class 
BjL 
Class 
g + e + d 
a 
h 
c 
h 
/+«■ 
a 
c 
q + c + d 
f+i 
h 
h 
3-00 
2-99 
2-97 
2-96 
2-96 
2-95 
a 
f+i 
c 
g + e + d 
h 
b 
72-64 
72-54 
72-30 
72-27 
71-95 
70-54 
Mean 
2-98 
Mean 
72-30 
The relationship is small, but exists. It seems reasonable to suppose that 
the order of mottling classes as given by B or B/L, where there is only one 
displacement, may be a better one than that we have selected. But if in the 
mottling order b and c were interchanged, it would agree with the B classification, 
in so far that the three classes of least and of most mottling in the two classi- 
fications would be the same. 
We now turn to the ground colour. We see that the ground colour is 
fainter, when the egg has greater breadth, but that there is no relation of the 
index to the intensity of ground colour. The results of p. 147 are thus confirmed 
by the general correlation of ground colour and breadth. Although there is no 
high-correlation, we may assert that it is probable that the intensity of pigment 
does not depend on the pressure during ti-ansit of the oviduct, but rather on 
a constant amount of pigment being distributed over a larger surface. 
(5) Homotijposis in Eggs of the same Clutch. 
The homotyposis, or degree of resemblance in character between eggs of the 
same clutch may be studied on the present material. The chief direct and cross 
homotypic correlations are given in Table IV. 
Pearson has shewn* that the degree of resemblance of undifferentiated 'like 
organs' might be expected to be equal to that of pairs of brethren, i.e. about '50, 
and proved that this is so for many hoinotypes in the vegetable kingdom, a result 
which has been since confirmed by much as yet unpublished material from the 
animal kingdom, including a number of series of birds' eggs. Thus the mean 
value of the homotyposis for eggs of the Common Tern could hardly be improved 
upon. Only the colour characters show irregularity, especially the mottling, a 
* " On Homotyposis in the Vegetable Kingdom," Phil. Traim. Vol. 197, A, pp. '285 — 379, 1900. 
