166 
The Cuckoo's Egg 
escape detection, it may be ejected. Moreover, the theory seems to demand that 
male Cuckoos should mate with female Cuckoos reared by foster-parents of the 
same species, or else that the inherited habits and characters of every female 
Cuckoo should follow only the female line of descent. For suppose a Reed-wren- 
reared female Cuckoo to mate with a Robin-reared male Cuckoo, then their 
offspring might be reasonably expected to inherit some characters from each 
parent and to possess mixed tendencies, some urging them to lay in Robins' nests 
and others in those of Reed-wrens, and, unless inheritance run only in the female 
line (or mating taking place only between individuals of like foster-parentage), 
the tendencies would get further mingled in each succeeding generation. This 
criticism appears to me to apply with equal force to Reh's theory of the intuitive 
selection of the nests of the species by which the Cuckoos themselves were reared. 
Further, there is very little, if any, evidence in support of the operation of natural 
selection in eliminating eggs that do not match those of the clutch into which 
they have been introduced. I have not come across any record of such badly- 
matched Cuckoo's eggs having been found ejected from the nest. Lastly, it is 
very difficult to conceive how perfection of colour-matching can have arisen by 
natural selection with relatively few opportunities for the working of this force; 
and, if the case be as Prof. Newton states, I certainly should have expected to 
find a large number of Cuckoo's eggs in the nests of those birds whose eggs had 
at length been so admirably copied. 
Size-Matching. 
In spite of these criticisms I am compelled by the results of ray investigations 
to admit that I now believe Professor Newton's theory to be in the main correct : 
I will return to this point later. It will be seen by the appended summary of 
results and tables of measurements that the mean length (22"40 mm.) and 
mean breadth (16"54 mm.) of 243 Cuckoo's eggs are respectively greater than 
the mean lengths and breadths of the eggs of any of the four species [viz. Anthus 
jyratensis, the Meadow Pipit; Anthus trivialis, the Tree Pipit; Accentor modularis, 
the Hedge Sparrow, and Erithacus rubecida, the Robin], of whose eggs I was 
able to measure a reasonable number. The range of length extends from 19"1 mm. 
to 25'0 mm. ; that of breadth from 14'0 mm. to 18"8 mm. : the standard deviation 
(o-) of length being 1'058, and the coefficient of variation (C. of V.) of length 4 72, 
those of breadth being respectively 0 6496 and 3"93. Of the four species which 
serve as foster-parents and are here dealt with, the Meadow Pipit alone exceeds 
the Cuckoo in degree of variation, but it must be remembered that the number of 
measured eggs of Robins and other species is very much less than in the case 
of the Cuckoo, so that it is quite possible that the Cuckoo does not differ greatly 
from other birds in this respect*. It is however interesting to note that in 
* Furtber, the Cuckoo's eggs were probably laid by about 200 separate hens, while the 74 Meadow 
Pipit's eggs, for example, are formed by 20 clutches or due to 20 hens only. 
