450 



MERISTIC VARIATION. 



[part I. 



indentation and is very nearly symmetrical about the rachis of 

 the feather, though each of its halves has no axis of symmetry. 



I II 



Fig. 140. I. Part of tail-covert of Polyplectron chinquis, with the two ocelli of 

 nat. size. II. Part of tail-covert of Polyplectron malaccense, with the two ocelli 

 partially confluent, of nat. size. 



(From C. Darwin, Descent of Man, 1871, n. p. 139, figs. 54 and 55.) 



Attention should be called to the fact that abnormal division along 

 a middle line may in many cases represent one of two different pheno- 

 mena which are not readily distinguishable. For when a normally 

 single organ is represented by two, standing on either side of a middle 

 line it is often possible that there may be not only a division of the 

 organ but a partial duplicity of the axis. These two conditions are of 

 course morphologically distinct ; for in the case of division of the organ 

 only, the two parts are still in symmetry about the original axis of 

 Major Symmetry of the body, but in the case of duplicity of the axis 

 there are two equivalent axes of symmetry, about which each half is 

 separately symmetrical. But though this distinction is in a sense a 

 real one it cannot be applied to cases of duplicity occurring in any 

 organ whose halves assume a bilaterally symmetrical form when sepa- 

 rate. For example in the case of the foot of the Horse, or of the 

 hjemal spines &c. of Gold-fishes (v. infra), when division occurs, each of 

 the two halves is only hemi-symmetrical, and this duplicity is no more 

 evidence that the axis is double than is the ordinary double condition 

 of the vertebrate kidney; but in the case of duplicity of the central 

 neural canal in Man for instance, or in the case of the tail-spine of 

 Limulus described below, it is not clear that there is not a partial 

 duplicity of the axis. 



Division or absence of union in Middle Line. 



Most of the organs which in a vertebrate stand in a median 

 position have been seen more or less often in a divided condition. 



