244 MR J. B. DOBBIE ON 
to which the eggs are exposed. But, as Prof. Newton has 
pointed out, “even if Lunel’s observations were correct, it 
matters not, for the ark-like structure of the nest remains 
constant.” It is impossible to discuss the question here, 
but it seems to me that a more reasonable solution of the 
difficulty is that this little bird, which, be it remembered, 
has an enormous geographical range, has sprung from a 
common ancestor with birds of the genus Cettia. Granting 
this, it is comparatively easy to account for the marvellous 
variability in the eggs, not only of the genus Cisticola, but 
also of those of closely-allied genera such as Orthotomus. A 
very much more difficult problem is, to my mind, furnished 
by the Sardinian Warbler, Sylvia melanocephala. This bird 
lays two totally distinct types of eggs, differing not only in 
colour but in size. The two varieties, which never seem to 
intergrade, are found commonly in the same districts, and 
with our present knowledge of the bird it is impossible even 
to guess the cause of this extraordinary variability. But we 
find variation in colouring most pronounced in the eggs of 
parasitical species, especially in those birds, such as Cuculus 
canorus, which place their eggs in the nests of a great many 
different species of birds. In the case of those parasites, 
such as Coccystes glandarius and Molothrus rufoaxillaris, whose 
dupes are confined to a few closely allied species, we invari- 
ably find that the eggs of both parasite and dupe closely 
resemble each other. On the other hand, the eggs of Cuculus 
canorus, Which “ exercises an indiscriminate choice in the 
selection of foster-parents for its young,” show in only two 
cases, according to the unique experience of Rey, any adap- 
tive similarity in colour to those of the dupe. This is, of 
course, strong proof of the soundness of Wickmann’s recent 
research. I have recently been forcibly struck by two eggs 
of parasitical birds, one of which is in my own collection, and 
the other in that of our distinguished member, Mr Davidson. 
The former, though a well-known authority thinks it belongs 
to Cuculus canorus, is probably that of Hierococeya spar- 
veroides, In any case it is that of a Cuckoo, and yet it 
strikingly resembles, even in the characteristic scribblings, 
the eggs of the rightful owner of the nest (Zmberiza ciopsis) 
from which it was taken. The other is the egg of Pentho- 
