428 DISCUSSION. 



individual climbs up its own genealogical tree, it may be safely 

 assumed that the ancestry of M . tiovce-hollajidice at one time did 

 not possess white irides. Furtlier, in this respect, M. sericea 

 would approximate more to the original common ancestor than 

 would M. novce-hollandiot, and thus would be like M. australa- 

 siana, the fifth species of the genus. The ancestors of the genus 

 Melio7'nis, like the other species of Meiiphagidce whose irides have 

 been examined, had, therefore, almost certainly, dark-coloured 

 eyes. Now it can hardly be maintained that the striking white 

 iris has been gradually evolved by a slow series of changes, each 

 of which was of particular service to the individual possessing 

 it, especially when there is seen, side by side with the species in 

 question, another descended from the same stock, but not showing 

 these changes. It seems more reasonable to assume that a 

 sudden marked mutation occurred in the development of a white 

 iris; and that, in the terms of Alendelian heredity, this feature 

 was a dominant one, and stamped itself effectively on the 

 offspring of the white-irised bird and its brown-irised mate. 

 These offspring, possessing dominant white-iris characters and 

 recessive brown ones, may be supposed to have bred more between 

 themselves — the birds being considered to be capable of recog- 

 nising the difference in colouring — than with brown-eyed fellows; 

 and so eventually, though occupying the same liabitat, a pure 

 strain of the white-eyed type arose. As time went on, and the 

 white-eyed birds mated with each other, to the exclusion of the 

 brown- eyed forms, and vice-versa, secondary differences such as 

 those of plumage and note evolved; until, finally, the present-day 

 species were firmly e.stablished. The example here quoted may 

 be taken as an instance of the mutational origin of a species. 

 That this is a highly probable explanation, will be admitted, 

 when the case of the famous Ancon breed of sheep is borne in 

 mind. This race, which finally bred quite true to itself, arose 

 as a "sport" in a single animal; the special character being a 

 dominant one, it impressed itself on the mixed offspring. But, 

 in the difference between the West and the East Australian 

 species of Meliornis, still another process is seen to be at work^ 



