420 HETEROCHROSIS 



elsewhere as A. flavkollis, — one form of which, A. gouldi, inhabits 

 Australia. 



The true Bitterns, forming the genus Boiaurus, have been 

 already noticed (pp. 40-42); and of the Night-Herons Schlegdl 

 recognized six species, all to be reasonably placed in the genus 

 Nydicorax, characterized by a shorter beak and a few other 

 peculiarities, among which the large eyes deserve mention. The 

 first is N. griseus, a bird widely spread over the Old World, and 

 not unfrequently visiting England, where it would undoubtedly 

 breed if permitted. The same author united with it the common 

 Night-Heron of America ; but this, though very closely allied, is 

 generally deemed distinct, and is the N. ns&vius or N. gardeni of 

 most \;a'iters. A clearly different American species, with a more 

 southern habitat, is the N'. violaceus or N. cayennensis, Avhile others 

 are found in South America, Australia, some of the Asiatic Islands, 

 and in West Africa. The Galapagos have a peculiar species, N. 

 pauper, and another, brevipennate and no doubt peculiar, iV. mega- 

 cephalus, existed in Eodriguez at the time of its being first colonized, 

 but is now extinct. To this section undoubtedly belongs the BOAT- 

 BILL (p. 45), though it deserves generic distinction as Cancroma.^ 



Bones of the common Heron and Bittern are not uncommon in 

 the peat of the East -Anglian fens. Remains from Sansan and 

 Langy in France have been referred by M. Alphonse Milne-Edwards 

 to Herons under the names of Ardea perpleza and A. formosa ; a 

 tibia from the Miocene of Steinheim by Dr. Fraas to an A. similis, 

 while Sir R. Owen recognized a portion of a sternum from the 

 London Clay (see above, i)p. 281, 282) as approaching this Family. 



It remains to say that the Herons form part of Prof. Huxley's 

 section Pelargomorphx, belonging to his larger group Desmognathx, 

 and to draw attention to the singular development of the patches 

 of " powder-down " which in the Family Ardeidds attain a magnitude 

 hardly to be found elsewhere. .Their use is utterly unknown. 



HETEROCHROSIS, the collective term signifying the occur- 

 rence of abnormal coloration which may be due to one or other of 

 various causes, such as 



1. The partial or total absence of pigment producing a paler 

 hue or even a complete Albino. 



2. The overproduction of pigment resulting in a more intense 

 tint or the introduction of a new colour. 



3. The absence of or change in the surface overlying the pigment. 

 There seems to be a certain correlation of colours in most cases 



of Heterochrosis : for instance, feathers with a yellow pigment 

 have a tendency toward orange and red ; green feathers exhibit 



1 The SuN-BiTTEUN {Eurypyga), by some systematists cousidered to belong to 

 the Ardeidas, certainly forms a Family by itself. 



