the Coloration of Eggs. 563 



a day or two of hatching. Meanwhile, I will perhaps not 

 be blamed too greatly for provisionally placing this mouth 

 and that of Cisticola erythrops amongst the arguments 

 against the nestling-adaptation view. I may add that an 

 unhatched Prima had the spots quite separate and un- 

 connected — which leads one to enquire whether the tongues 

 shown in my text-figure were not perhaps specially advanced 

 rather than reversionary — and that a younger embryo of 

 Heliolais, the mother of which had not lost the twin spots, 

 had its mouth already tinged with yellow, but (so far as 

 I could see) no spots. Those Prinia embryos nearly as 

 advanced showed no spots, but I am not sure that they 

 were yet beginning to show any pigmentation at all. 



I have already, in a footnote, referred to the fact that the 

 substitution of a nestling with a spotless mouth for one 

 possessing the twin spots strongly developed, did not to all 

 appearance result in any detriment to the former or in 

 trouble to the latter's parent. I have also specially tested 

 the rest of Mr. Pycraft's view, that mouth-spots "occur 

 just in those areas where the mouth is most sensitive to 

 touch, so that they serve a double purpose — they form a 

 guide to the parent, and ensure a mechanical closing of the 

 mouth directly the right stimulus, given by the touch of 

 solid food, is administered.'^ Mr. Pycraft applies this view 

 even to nestlings " hatched in open nests, on the ground, 

 amid short grass,'' believing that the spots " were probably 

 developed before the birds adopted this more open nesting- 

 site ; and, this being so, ... . they are still needed to serve 

 as a cue, so to speak, to the right co-ordination of move- 

 ments necessary for the sure transference of food to its 

 destination." 



The view may quite likely apply in its entirety to cases 

 I have not tested. I have experimented on very many 

 nestlings, however, mostly with the twin-spot tongue and in 

 covered nests, with the following general result : — (1) the 

 apical portions of the mouth, including the tip of the 

 tongue, are less sensitive than the inner portions. Mouth- 

 spots occur in both areas. (2) Of the more sensitive parts, 



2q2 



