BRITAIN'S BIRDS AND THEIR NESTS. 223 



carried about till a clutch was found to match it. But 

 this, as we have seen, is not reconcilable with what we 

 know of the habits of the Cuckoo. That the nest is 

 chosen first seems to be the usual, if not the invariable, 

 rule. That any control can be exercised through mental 

 channels on the coloration of the egg about to be laid 

 is difficult to believe, although theories on these lines 

 have been put forward. More readily can we accept 

 the hypothesis that each individual Cuckoo lays a paHicular 

 and constant type of egg throughout its life, and possesses, 

 correlative with this, the instinct of selecting nests of 

 foster-parents suitable to this type. That both instinct 

 and egg-type are hereditary, perhaps entirely through the 

 female line, is an almost necessary extension of the 

 theory. Thus we should have ' Titlark-Cuckoos,'' ' Wag- 

 tail-Cuckoos,"* and so on — ' varieties '' of the Cuckoo species 

 differing not in plumage or structure, but in nest- 

 selective instinct and egg-type. Taking each ' variety ' 

 separately, the phenomena now come within the scope 

 of the ordinary theories of adaptation. 



This hypothesis is not without objections, but it is 

 perhaps not far wide of the mark. Certainly it is the 

 most reasonable one in the light of our present knowledge, 

 but there is still plenty of scope for investigation. 



The number of eggs laid by any individual Cuckoo 

 in a season is naturally difficult to determine. Some 

 observers compute it at six or seven ; others considerably 

 higher. Let us put aside the question and note the 

 subsequent history of a single egg. 



The small size of the egg in proportion to the bird serves 

 another purpose, as well as that of making it resemble 

 the eggs of the small foster-parents. Shortage of food- 

 supply within the shell probably hastens the date of 

 hatching, for we find that the incubation-period is as short 



