106 Proceedings of the Royal Physical Society. 
Close resemblances between the offspring of a single bird are more 
to be expected than between those of several. From such a conclusion 
it. follows :-— 
Cuckoos laying in Sedge Warblers’, Reed Warblers’, Whitethroats,’ 
Lesser Whitethroats’, Reed Buntings’, Robins’, Hedge Sparrows’, and other 
named birds’ nests do not in England form separate gentes, “each with its 
own particular habits in respect of selecting a foster parent.” 
It may, of course, be pointed out that it is not held that individuals 
of a supposed “gens” adhere in every case to the gens type of foster nest, 
but only “in the main.” This means, however, that the departure from the 
“gens” habit is exceptional, which cannot be claimed with regard to the 
cases under consideration. In all these the foster species are multiple, 
except one—(I.) a case of two eggs in Reed Warblers’ nests. 
An important factor bearing upon the problem of the evolution of 
Cuckoo “‘gentes” exhibiting specialised egg characters is the polyandric habit 
of the Cuckoo. If we allow that the female exhibits a hereditary bias in 
the selection of foster parents for her offspring which is evolving along with 
a definite morphological differentiation in egg dimensional characters, we 
exclude the male parent, who may be of a different gens, from exercising 
any hereditary influence upon the “foster-bias” of the offspring, with its 
accompanying egg differentiation. To put the matter concretely : if a female 
bird bred from a “ Robin-cuckoo” gens mates successively with a male of 
“ Hedge-sparrow-cuckoo ” stock and with a male of, say “ Wren-cuckoo” 
stock (assuming that such stocks exist), will the offspring of these matings 
be “ Robin-cuckoos” only or even mainly? The “gens-theorists” must have 
an answer to this problem ? 
For the view of a possible evolution of “gentes” going on, it may be 
stated: Cuckoos may come back not only to the locality but to the 
particular hedge or reed bank in which they were reared, in search of a 
foster parent for their offspring. It is possible that they are predisposed— 
some more, some less—to observe more readily, and so to find the species 
and the nest of the type which incubated and reared them. There may 
be part passu with this, a process of Natural Selection going on whereby 
an increasing number of Cuckoos reared by foster species of a specially 
assiduous and faithful type are surviving, and in turn a majority of these, 
reminiscent of and let us hope grateful for past privileges, which tend to 
seek a similar foster parent for their own offspring. In some such way 
physiological sub-species may be in process of forming. 
This theory is plausible enough, and there may be some support for it 
in the frequency with which in some districts a particular type of foster 
