Oswald H. Latter 
167 
each of the five species length of egg is a far more variable dimension than 
breadth : this is probably due to uniformity of diameter of the oviduct, and it 
may also be of importance to the comfort of the female bird during the period 
of incubation, for an egg projecting far above its fellows in consequence of greater 
breadth would probably inconvenience the sitter. 
My enquiry has thus resolved itself chiefly into an attempt to ascertain 
(1) if the eggs of Cuckoos deposited in the nests of any one species stand out as 
a set apart from Cuckoo's eggs deposited elsewhere ; (2) if the same eggs depart 
from the rest in such a direction as to approximate in size to the eggs of that 
particular species of foster-parent. The metliod employed is to find the mean 
(M) length or breadth, as the case may be, thence to compute the standard 
deviation (a) by the formula a-- = ^^^"^ — — , where a; = the measurement of 
any one egg and n = the number of eggs measured : and then to find ^^"^ , the 
coefficient of variation. To test whether any deviation is significant, il/,. is taken 
as the mean of the whole race of Cuckoos and Ms the mean of Cuckoo's eggs found 
in the nest of any one species of foster-parent : the standard deviation (o-.,) of such 
eggs is also ascertained. The value of — Ms is then compared with that of 
0 67449 \/ — + — , where -Hj = total number of Cuckoo's eggs and = the number 
of Cuckoo's eggs in the nests of the species in question, which is the probable 
error of M^ — Ms due to random sampling. If the value of il/,. — Ms be not at 
least r5 to 3 times as great as the value of the other expression the difference of 
Mr and Mg is not definitely significant. Referring now to the tabulated summary 
below, it will be seen that in the matter of length, the eggs of Meadow-Pipit- 
Cuckoos (to coin a convenient phrase), of Wagtail-Cuckoos (this includes all species 
of Wagtails, for their eggs are very similar), and of Robin-Cuckoos, do not differ 
significantly from those of the whole race of Cuckoos, but those of Hedge-Sparrow- 
Cuckoos, Tree-Pipit-Cuckoos and of Wren-Cuckoos certainly do present differences 
marking them out as distinct sets. On the other hand in the matter of breatlth 
the differences are significant in the cases of Meadow-Pipit-Cuckoos, Hedge- 
Sparrow-Cuckoos and Wren-Cuckoos. It therefore seems highly probable that there 
are certain " gentes " of Cuckoos whose members being closely related lay eggs 
of somewhat similar dimensions and in the main confine their attentions each 
to its own particular variety of foster-parent. Breadth, as I have already pointed 
out, seems more likely than length to be a disturbing factor in the nest of the 
foster-parent if it in any marked way depart from the normal, and it is very re- 
markable to note how very low are four of the values of as in the breadth tables, 
viz. 2-28 (Meadow-Pipit-Cuckoo), 2-58 (Wagtail-Cuckoo), 2-65 (Tree-Pipit-Cuckoo), 
1-92 (Wren-Cuckoo). 
Next, as to whether these sets differ from the main body in the sense of 
the particular species of foster-parent. In the Wren-Cuckoos this is so beyond 
