1917-] Letters, Extracts, and Notes. 269 



In nestliyigs' mouths it seems to be also a matter of relative 

 acceleration. Thus, allowing for cases of suppression (which 

 are betrayed sometimes h\ incompleteness), the mouths of 

 Passerine birds of various families run through what appears 

 to be the same succession of changes, but particular stages 

 coincide with different tiges in different species, and some- 

 times even in individuals of the same species, producing 

 inter-specific diversity and intra-specific variation and even 

 the appearance of " mutation." It was in this sense that I 

 used the term " reversionary " of the unusual Warblers* 

 tongues of my text-figure (' Ibis,' 1916, p. 293), and I feci 

 that it would have been exceedingly interesting had 

 Capt. Ingram been able to see one or more of his aberrant 

 Skyhark months ('Ibis,' 191G, p. 523) before hatching, 

 and keep track of the post-nidal development of the others, 

 and to note that the plain yellow tongue was preceded or 

 succeeded by a twin-spot stage. I must here qualify a 

 statement, made in the present connection on p. 562, with 

 regard to an incipient resemblance between a stage passed 

 right through by the mouth of Dryoscopus guttatus and the 

 final month of Batis, &c. The darkening never became 

 more than incipient, and may or may not have been of 

 significance. The real blackening, when it began, was 

 centrifugal. 



In applying this view to eggs — here it might be suggested 

 that the successive processes of pigment deposition consti- 

 tute an ontogenetic phenomenon — I used as illustrations 

 especially figs. 5, 24, 25, and 20 ('Ibis,' 1916, pi. xix), 

 representing sufldciently four forms of egg laid by members 

 of the genus Coss?/pha. The egg of C. natalensis, normally 

 at Chirinda the dark brown nightingale-like egg of fig, 20, 

 is sometimes found with fewer pigment-layers (apparently, 

 like fig, 24) and, rarely, as a pale blue e^^ like the Red- 

 start's, Each form represents what is the usual egg at this 

 moment, of others of the Turdidse and is found also in 

 other families. Conversely, C. heugliai, laying normally at 

 Chirinda an egg more or less like fig. 25, was once found 



