VARIATION 299 



Further, and this is especially interesting, the allied Pacific 

 Guillemot (Lomvia californicd) also produces a precisely similar 

 variety. 



The Ruff {Machetes pugnax) when in summer dress, at any 

 rate, affords an even more striking case. It is a matter of 

 common knowledge that the broad frill and occipital tufts ol 

 feathers, which are assumed by the males during the period of 

 courtship, present the most wonderful range of coloration, no 

 two birds being exactly alike, while the extremes of coloration 

 and pattern are most striking ; but the peculiarities of each 

 bird appear to be faithfully reproduced annually. The ruff 

 and ear-tufts however are not the only portions of the plumage 

 which thus set at defiance, so to speak, the general observance 

 among birds of conformity to type, for the body plumage varies 

 just as widely, though this fact is generally lost sight of But 

 however the coloration may vary, the general proportion of 

 this ruff and " ear-tufts " remains constant. The females, how- 

 ever, sombrely clad, present a wonderful uniformity. 



More impressive are the facts with regard to certain species 

 of small Herons, which presenting two distinct phases — a white, 

 and a dark phase — were for a long time regarded as so many 

 distinct species. Perhaps the best-known instance of this 

 curious fact is that furnished by the Reef-heron {Demigretta 

 sacra). The typical plumage of this bird is of a deep blackish- 

 slate, but wholly white birds are common. It does not appear, 

 however, to have been established whether these white birds 

 ever occur where both parents are of the dark type. Blue and 

 white birds have been seen paired together, and the offspring 

 in such cases seem to be more or less intermediate in coloration, 

 being streaked with blue. It is significant to note that the 

 dark forms are darker in winter, and darkest when immature, 

 the adults being much paler in colour. 



But this remarkable dimorphism is by no means confined to 

 this species, and it seems highly probable that we have in these 

 instances an illustration of the lines of evolution which will 

 ultimately end in suppression of the dark and the survival of 

 the white forms. The evolution of the allied White Egrets has 

 probably followed precisely similar lines. 



The following facts seem, at any rate, to support this in- 

 terpretation ; — 



