334 On the Nest and Eggs of the Common Tern 
thus ifc is exceedingly improbable that the cross-homotyposis could be significant*. 
Direct homotyposis provides the results of the following table : 
TABLE XX. 
Direct Homotyposis in Mottling and Ground Colour Value. 
Character 
Season 1913 
Season 1914 
Mottling of Egg.s in same Clutch 
Ground Colour Value of Eggs in same Clutch 
•3500 
■5709 
•6267 
•7480 
The probable errors of the 1913 values are well below "045 and of the 1914 
values well below "017. Accordingly the differences are markedly significant, or in 
the nature of pigmentation the resemblance of eggs in the same clutch is much 
more intense in 1914 than in 1913. Thus the results for size and shape of egg are 
confirmed by those for pigmentation. We have therefore this very remarkable 
fact — a fact which it seems to us may be of some consequence — namely that the 
season can affect the extent to which the female bird impresses her individuality 
on the external characters of the egg. It does not follow from this that seasonal 
differences can affect in the like marked manner the individuality of the internal 
characters of the egg. But it does raise the suggestion that it would be well 
worth inquiring whether the degree of resemblance of offspring born in one 
season can differ sensibly from the degree of resemblance of those born in another 
season. Should such a difference be established, it would indicate that heredity — in 
other words the nature of the germ plasm — could be more readily influenced by 
seasonal differences than has yet been anticipated. We ourselves should be very 
unwilling to admit this, but we must at the same time confess that we see no 
obvious explanation of these significant changes in homotyposis. If individuality 
impressed in the ovary and in the oviduct on the form and colouring of eggs can be 
increased or decreased by seasonal differences, it is not a very long step to believe 
that other physiological processes of this region which impress individuality on the 
internal characters of the ovum can be modified by the nature of the season. 
We now turn to the cross-hoinotyposis in size and shape of the tern's egg : 
TABLE XXI. 
Cross- Hoiuotyposis in Size Characters. 
Characters of the two Eggs 
Season 1913 
Season 1914 1 
Length and Breadth... 
Longitudinal and Transverse Girths 
Length and Longitudinal Girth ... 
Breadth and Jjongitudinal Girth ... 
•0922 + ^0441 
•2603+^0413 
•4229+ ^0362 
•2530+ ^041 6 
•2621 + -0157 
•4546+ -0134 
•5854 + -0111 
•4162+ •0140 
* See above our second footnote on p. 328. In 1913 we had not fully realised how high C2 could 
be for such short samples as a couple of hundred. Hence the source of the error. 
