Orthogenetic Characters. 103 



CHAPTER XII. 

 ORTHOGENBTIC CHARACTERS. 



In previous chapters we have drawn a contrast between (a) 

 karyogenetic characters originating as mutations in the germplasm 

 and affecting every nucleus, and (b) organismal characters which 

 belong to the organism as a whole, show recapitulation, and 

 probably originate as environmentally impressed modifications of 

 the cytoplasm. In this chapter we wish to consider briefly 

 orthogenetic characters, which appear to stand midway between 

 these two categories. Like the former they are germinal in origin, 

 and like the latter they show recapitulation. But they differ from 

 the recapitulatory characters previously considered, in that their 

 origin is apparently not adaptational but on the contrary 

 independent of environment. We will refer only to three cases 

 from the recent experimental work with animals. 



It is a well-known fact that in many birds the juvenile plumage 

 differs from that of the adult, and usually at least represents a less 

 specialized and presumably ancestral type of plumage. One of 

 the most striking cases, recently studied by Beebe (1914), is in the 

 white ibis, Guam alba. In the young chick the head and neck are 

 covered with black down, becoming smoky gray over the greater 

 part of the body except the under parts, which are white. In the 

 juvenile and post-juvenile stages this is gradually replaced by white 

 feathers, until late in the second year the birds are pure white with 

 scarlet legs and bill. It seems clear that such a white bird has not 

 originated through a mutation, and that the ontogeny represents a 

 gradual transition from a dark-coloured ancestor. It is an 

 interesting fact that the white loral spots in the young chick, 

 which quickly disappear, apparently represent the permanent facial 

 marking of a related ibis, Plegadis autumnalis. This would appear 

 to be one of the few cases in which a specific (generic) difference is 

 at the same time a recapitulatory character. 



Whitman's (1919) recent posthumous volumes on orthogenetic 

 evolution in pigeons cite a number of cases of juvenile plumage as 

 recapitulatory stages furnishing evidence of orthogenetic develop- 

 ment. Whitman's study of the wing patterns of pigeons is perhaps 

 the most prolonged and intimate investigation which has ever been 

 made of a single character. Reading the series in the opposite 

 direction from Darwin, he concluded that the primitive condition 

 was a uniform chequered pattern covering the whole wing, as in 



