146 
On Homotyposis in Eggs of the Common Tern 
The mottling is a far more difficult matter for determination. The points 
which may be considered ai'e : 
(i) Size and shape of individual splodges. 
(ii) Portion of the egg over which these splodges are distributed. 
(iii) Area of mottled surface as compared with whole area of the egg. 
The fieldworkers selected 9 typical mottlings (see Plate IX) and named these 
a, h, c, d, e, f\ g, h, i ; they then compared each recorded egg with these and selected 
the letter which marked the egg on the scale most resembling the egg to be 
recorded. There is little doubt that in this manner they divided the whole series 
of eggs into differentiated classes. But it may be doubted whether the judgment 
made depended on one only of the above three characteristics. Hence when we 
came to arrange the eggs a, b, c, d, ... h, i on a scale of mottling, we found that the 
order would not be the same when we classified in turn by eacii of the three 
characteristics. We endeavoured to place the eggs in order by extent of mottling, 
i.e. by (iii), but we think that the relatively low value of the homotyposis which 
has resulted is possibly due to size and shape of the mottlings, (i), having had 
as much influence on the classification as the extent of area mottling. Even 
position on the egg, (ii), can influence judgment considerably. We believe that 
in futui-e work on eggs, it would be desirable to classify the mottling of each 
by using the three characteristics independently. Even then an ocular appre- 
ciation, as this must be, may fail to give a very close measure of the nature of the 
mottling and thus weaken any homotypic correlation. 
The Ground Colour of these eggs varies through all shades of brown to 
brownish greens and blue-greens. The fieldworkers attempted to give the value or 
depth of ground-colour pigmentation without regard to the brown or green shade 
of colouring. The scale of values is given at the foot of Plate VIII. 
A point seemed worth consideration : assuming the pigments to be deposited 
on the egg in its passage through the oviduct, it was conceivable that greater 
pi'essure might indicate greater intensity of pigmentation. We accordingly 
selected the broader egg in each clutch and investigated for every pair of eggs 
from the same clutch whether the broader or narrower egg had the larger mass 
of mottling and greater density of gi'ound colour. We reached the following 
results : 
The broader egg in every possible clutch-pair has 
Greater mottling in 26 cases More dense ground colour in 25 cases 
The same „ 37 „ The same „ „ 39 ,, 
Less „ 40 „ Less dense „ „ 37 „ 
Perhaps not very much stress is to be laid on these results, but they suggest 
that the total amount of pigment deposited is less the broader the egg, i.e. for the 
same bird a relatively smaller egg will be more pigmented. A solution of this 
