O. H. Latter 
367 
of the foster-parents' eggs and those which the Cuckoo lays in their nest. It 
is worth while considering this relationship from another standpoint. The bulk 
of an egg must be approximately proportional to its length x (breadth)^ and 
we may take this roughly as a bulk-modulus. Using it we obtain the foregoing 
Table II. for the bulk-moduli in cubic millimetres. This leads to a correlation 
of '1746, or "17, say, and confirms the previous results. There thus appears to 
be really a small relation between the bulk of the Cuckoo's egg and that of the 
foster-parent. The results are based on only 16 classes, which is, of course, 
small for determining a correlation, but it would seem that we must, since all three 
measurements give a positive correlation, assume some slight relationship between 
size of Cuckoo's egg and that of the foster-parent. This relation might be 
clearer and less irregular, if we had measurements of fairly long series of the eggs 
of the foster-parents. It is perhaps worth noting that the breadth measurement is 
the one in which the resemblance is closer. We may thei-efore ask : Is it possible 
that the differentiation in size of egg, which we find as we pass from the foster- 
parent of one to the foster-parent of a second species, is merely due to a slight 
constriction of the oviduct, instinctive or otherwise, following on the choice of a 
foster-parent ? Wei'e this the case, it would be unnecessary to suppose separate 
Cuckoo gentes. Now the relation between length and breadth of egg has been 
worked out for Thrush, Blackbird, House-Sparrow and Linnet and the value of the 
correlation lies between 'Zb and '40. We should expect therefore that if the 
differentiation of the Cuckoo's egg were produced solely by such a constriction of 
the oviduct, that the length of the egg would be increased, or the above correlation 
between length and breadth be at any rate reduced. W^e find, however, that if the 
length and breadth of the 16 Cuckoo groups be correlated, the resulting value is 
'91. There can hardly be a doubt therefore that the variation in the Cuckoo's egg 
as we pass from one species of foster-parent to a second is a real variation in bulk, 
the length and breadth changing in a closely correlated manner. Thus although 
the egg is only slightly correlated with the size of the foster-parent's egg it varies 
as a whole in a manner not consonant with a mere constriction of the oviduct, and 
the fact that the breadths of foster-mother's and Cuckoo's eggs are more closely 
correlated than their lengths, does not enable us to dispense with a theory of 
gentes. 
Table IV. shows the dimensions of the eggs of the foster-parents for comparison 
with those of the Cuckoo frequenting their nests. The approximate sizes of the 
birds are also inserted. 
(3) On the Distribution of Frequency in the Cuckoos Egg. 
A consideration of the first two columns of totals under the heading '■' Race" in 
Table III., shows that Rey has measured the great bulk of his eggs to only '5 mm. 
accuracy; he has probably not used a vernier and the result is that his frequencies 
are crowded up on the whole and half millimetres; a similar crowding although 
less conspicuous is obvious in the additional 7 17 measurements. This unconscious 
