172 
The Cuchoo's Egg 
evidence of the ejection of Cuckoo's eggs by small birds, I feel compelled to 
admit that thei'e is a selective process at work, tending, in many cases, to bring 
the Cuckoo's eggs into agreement with those of the host both in size and 
colour, and am inclined to suggest that perhaps there may be local " gentes " 
of Cuckoos which as a rule, but by no means exclusively, patronise the nests 
of particular species. It seems well established (1) that a Cuckoo returns every 
year to the same locality, and (2) according to Reh, lays its eggs only in the 
nests of that particular species which it, or its ancestors, happen to have 
adopted for that purpose, while the coloration of the egg of every female 
Cuckoo is peculiar to itself The evidence that my material furnishes on these 
points is as follows : — the Cuckoo's eggs in the South Kensington Collection 
from any one locality frequently exhibit strong resemblances inter se, e.g. 
(a) Nos. 107, 108, 109 (Robin-Cuckoo), 180 (Greenfinch-Cuckoo), 189 (Spotted- 
Flycatcher-Cuckoo), and 61 (Meadow-Pipit-Cuckoo), all taken at Churt in the 
year 1860, are all so closely similar that they may well be from one and the same 
bird; (b) Nos. 25 and 26 (Meadow-Pipit-Cuckoo) from Lochend, but not dated; 
(c) Nos. 34 and 35 (both in same nest of Meadow-Pipit) from S.W. Lancashire, 
dated June 29, 1866, and 33 (Meadow-Pipit-Cuckoo) from N.W. Cheshire, dated 
25 May, 1866, and all three collected by H. E. Smith ; (c^) No. 193 (Red-backed- 
Shrike-Cuckoo, June, 1863), 110 (Robin-Cuckoo, June, 1863), 195 (Skylark- 
Cuckoo, May, 1862), 181 (Greenfinch-Cuckoo, 1864), 243 (Linnet-Cuckoo, June, 
1864), and 168 (Yellow-Ammer-Cuckoo, June, 1864), all coming from Churt, 
and (e) Nos. 225—236 (Wren-Cuckoo), Pomerania, 1879, 1880 and 1881, exhibit 
the same phenomenon in their several sets. But it is to be observed that eggs 
of the same coloration are not confined to the nests of any one species of 
foster-parent, except in (c) and (e). Again, eleven nests contained two Cuckoo's 
eggs apiece, viz. Nos. 28 and 29, 34 and 35, 36 and 37, 38 and 39, and two 
others not accessible for measurement, all in Meadow Pipits' nests, 139 and 140 
in Orphean Warblers', 210 and 211 in Pied Wagtails', 214 and 215, 217 and 
218 in Pied Wagtails', and 145 and 146, 147 and 148 in Reed Warblers', and 
in each case the two eggs are obviously laid by the same parent. Reh mentions 
the fact that in 1893 within two kilometres from Leipzig no less than 70 nests 
were found containing Cuckoo's eggs, and of these, 58 (83 "/o), were in nests 
of Red-backed Shrike. It seems possible then that in any given locality a 
majority of the Cuckoos may favour some one particular species of foster-parent, 
and if this be so the chances of male and female Cuckoos of like rearing mating 
together are very largely increased, and the difficulty raised in a preceding passage 
is to some extent removed. It is however evident that the isolation of "gens" 
from " gens " is not perfect, and this may perhaps be accounted for by occasional 
matings between birds of unlike foster-parentage and the offspring inheriting 
mixed tendencies. 
In conclusion I must acknowledge my great indebtedness to Professor Karl 
Pearson for the interest he has taken in this investigation and for much kind 
advice and assistance in the statistical portions. 
