156 ORTHOGENETIC EVOLUTION IN PIGEONS. 



period of lowest vascular pressure, and therefore with the period of least favorable 

 nutrition. These, then, are at the basis of the production of the fundamental bars. 

 The growth-rhythms occur equally in feathers with and without pigment; the light 

 and dark fundamental bars are of course obtained only in (melanin) pigmented 

 feathers, and there, conspicuously or usually, only when a temporarily or perma- 

 nently unfavorable nutrition or metabolism in the organism is superposed upon the 

 nutritive fluctuations of the daily rhythm. 27 



2 ' Since it is the division or separation of a pigmented area that gives rise to fundamental bars (from which, accord- 

 ing to Whitman, "specific characters have been evolved") the question of the origin of the melanin pigment concerned 

 is here not the point of primary importance; I have, however, elsewhere (Biol. Bull., vol. 16, 1(109) discussed this topic, 

 and have pointed out that the formation of melanin lies close to generalized protoplasmic functions, rather than to 

 intricately specialized and segregable ones. — Ed. 



Note. — The manuscripts and records used in the preparation of this chapter were found in folders designated 

 as follows: BB 10, G 10, G 12, G 13, G 23, 0011, 0013, R 10, W 2, W 12, WW 1, WW 0, XG 21, XG 20, XW 2.— Ed.). 



Explanation of Plate 74. 



Fundamental bars in wing of juvenal Jacobin. Toda del., Aug. 1904. Natural size. 



This bird is black with red bars crossing the tertials and long and median coverts. 



The parents are red, with white head, tail, and primaries. The young has white parts, the same as the parents, 

 whose red is replaced by black in this young. This black is pure black, without the transverse barring, in the adult 

 feathers, a few of which (small coverts) are already present and shown in the drawing. The feathers of the later 

 development, therefore, are free from transverse bars and are blacker than the first feathers. 



The parents have produced such young twice this season (1904). I have black barbs (Jacobins) that produce 

 usually black young, but occasionally red, and in one case a gray. 



Explanation of Plate 75. 



Barred secondaries and long coverts of a juvenal Jacobin, age 8 weeks. Toda del., Feb. 1905. x 0.8. 



This is a second young (pi. 86 being the first). 



Feathers plucked August 25, 1904. The parents were both red and white, as described under plate 86. 



These bars are narrower above than below, and stronger in the exposed part of the feather — i.e., they weaken 

 gradually as we pass towards the inner end. The inner or basal half of the feathers shows only very obscure bars, 

 and these are hard to see. 



In the secondaries, reckoning 6 feathers, the barring is very obscure — hardly noticeable. The artist has exagger- 

 ated somewhat. In the tertials — inner (upper) eight feathers — the bars are strong only in the terminal third. 



In the second feathers the black prevails to such an extent that I should describe it as a black Jacobin (white 

 head, tail, and primaries). No bars are visible in adult feathers. 



